diff --git a/examples/benchmark_reduce_mime_encoding.phps b/examples/benchmark_reduce_mime_encoding.phps
new file mode 100644
index 0000000000..38489f83c7
--- /dev/null
+++ b/examples/benchmark_reduce_mime_encoding.phps
@@ -0,0 +1,230 @@
+<?php
+/**
+ * This example shows sending mail per receiver and reduce MIME encode.
+ */
+
+//Import the PHPMailer class into the global namespace
+use PHPMailer\PHPMailer\PHPMailer;
+
+//SMTP needs accurate times, and the PHP time zone MUST be set
+//This should be done in your php.ini, but this is how to do it if you don't have access to that
+date_default_timezone_set('Etc/UTC');
+
+require '../vendor/autoload.php';
+
+//Sample Images from https://sample-videos.com/download-sample-jpg-image.php
+// $ curl https://sample-videos.com/img/Sample-jpg-image-1mb.jpg > /tmp/1mb.jpg
+//
+$image_files = array(
+    '1.8kb' => 'phpmailer_mini.png',
+    '5.7kb' => 'images/phpmailer.png',
+    '50kb' => '/tmp/50kb.jpg',
+    '100kb' => '/tmp/100kb.jpg',
+    '200kb' => '/tmp/200kb.jpg',
+    '500kb' => '/tmp/500kb.jpg',
+);
+
+$receiver_count = 200;
+
+$receiver_list = [];
+for ($i=0 ; $i<$receiver_count ; ++$i)
+    $receiver_list[] = 'whoto'.$i.'@example.com';
+
+echo "Genrate $receiver_count receivers: " . count($receiver_list) . "\n";
+
+$results = [];
+$test_count_per_run = 10;
+
+echo "Sending $receiver_count mails with creating a new PHPMailer instance:\n";
+{
+    foreach ($image_files as $filesize => $image_file) {
+        echo "\tWith $filesize attachment, path = $image_file\n";
+        $total_time = 0;
+        for ($i=1; $i<=$test_count_per_run ; ++$i) {
+            $start = microtime(true);
+            foreach ($receiver_list as $receiver) {
+                $mail = new PHPMailer;
+                $mail->isSMTP();
+                $mail->SMTPDebug = 0;
+                $mail->Host = 'mail.example.com';
+                $mail->Port = 25;
+                $mail->SMTPAuth = true;
+                $mail->Username = 'yourname@example.com';
+                $mail->Password = 'yourpassword';
+                $mail->setFrom('from@example.com', 'First Last');
+                $mail->addReplyTo('replyto@example.com', 'First Last');
+                $mail->addAddress($receiver);
+                $mail->Subject = 'Hi '.$receiver.'!';
+                $mail->msgHTML(str_replace( '<h1>This is a test of PHPMailer.</h1>', '<h1>Hi '.$receiver.'</h1>', file_get_contents('contents.html')), __DIR__);
+                $mail->AltBody = 'This is a plain-text message body for '.$receiver;
+    
+                //Attach an image file
+                $mail->addAttachment($image_file);
+    
+                //Build the mail content
+                $mail->preSend(); 
+                unset($mail);
+            }
+            $cost = microtime(true) - $start;
+            $total_time += $cost;
+        }
+        $avg_time = $total_time / floatval($test_count_per_run);
+        echo sprintf("\t\ttest $test_count_per_run runs, avg. time: %.5f seconds\n", $avg_time );
+    
+        if (!isset($results[$filesize]))
+            $results[$filesize] = [];
+        $results[$filesize]['method1'] = $avg_time;
+    }
+}
+
+echo "Sending $receiver_count mails with only one PHPMailer instance: \n";
+{
+    foreach ($image_files as $filesize => $image_file) {
+        echo "\tWith $filesize attachment, path = $image_file\n";
+        $total_time = 0;
+        for ($i=1; $i<=$test_count_per_run ; ++$i) {
+            //echo "\t\tRun $i: ";
+            $start = microtime(true);
+
+            $mail = new PHPMailer;
+            $mail->isSMTP();
+            $mail->SMTPDebug = 0;
+            $mail->Host = 'mail.example.com';
+            $mail->Port = 25;
+            $mail->SMTPAuth = true;
+            $mail->Username = 'yourname@example.com';
+            $mail->Password = 'yourpassword';
+            $mail->setFrom('from@example.com', 'First Last');
+            $mail->addReplyTo('replyto@example.com', 'First Last');
+
+            foreach ($receiver_list as $receiver) {
+                       $mail->clearAddresses();
+                $mail->addAddress($receiver);
+                $mail->Subject = 'Hi '.$receiver.'!';
+                $mail->msgHTML(str_replace( '<h1>This is a test of PHPMailer.</h1>', '<h1>Hi '.$receiver.'</h1>', file_get_contents('contents.html')), __DIR__);
+                $mail->AltBody = 'This is a plain-text message body for '.$receiver;
+    
+                //Attach an image file
+                $mail->addAttachment($image_file);
+    
+                //Build the mail content
+                $mail->preSend(); 
+            }
+            $cost = microtime(true) - $start;
+            //echo "$cost sec\n";
+            $total_time += $cost;
+        }
+        $avg_time = $total_time / floatval($test_count_per_run);
+        echo sprintf("\t\ttest $test_count_per_run runs, avg. time: %.5f seconds\n", $avg_time );
+    
+        if (!isset($results[$filesize]))
+            $results[$filesize] = [];
+        $results[$filesize]['method2'] = $avg_time;
+    }
+}
+
+echo "Sending $receiver_count mails with creating a new PHPMailer instance and ReduceMIMEEncodeCacheStore: \n";
+{
+    foreach ($image_files as $filesize => $image_file) {
+        echo "\tWith $filesize attachment, path = $image_file\n";
+        $total_time = 0;
+        for ($i=1; $i<=$test_count_per_run ; ++$i) {
+            //echo "\t\tRun $i: ";
+            $start = microtime(true);
+            $cacheLookupTable = [];
+            foreach ($receiver_list as $receiver) {
+                $mail = new PHPMailer;
+                $mail->isSMTP();
+                $mail->SMTPDebug = 0;
+                $mail->Host = 'mail.example.com';
+                $mail->Port = 25;
+                $mail->SMTPAuth = true;
+                $mail->Username = 'yourname@example.com';
+                $mail->Password = 'yourpassword';
+                $mail->setFrom('from@example.com', 'First Last');
+                $mail->addReplyTo('replyto@example.com', 'First Last');
+                $mail->addAddress($receiver);
+                $mail->Subject = 'Hi '.$receiver.'!';
+                $mail->msgHTML(str_replace( '<h1>This is a test of PHPMailer.</h1>', '<h1>Hi '.$receiver.'</h1>', file_get_contents('contents.html')), __DIR__);
+                $mail->AltBody = 'This is a plain-text message body for '.$receiver;
+    
+                //Attach an image file
+                $mail->addAttachment($image_file);
+
+                  $mail->ReduceMIMEEncodeCacheStore = &$cacheLookupTable;
+    
+                //Build the mail content
+                $mail->preSend(); 
+                unset($mail);
+            }
+            $cost = microtime(true) - $start;
+            //echo "$cost sec\n";
+            $total_time += $cost;
+        }
+        $avg_time = $total_time / floatval($test_count_per_run);
+        echo sprintf("\t\ttest $test_count_per_run runs, avg. time: %.5f seconds\n", $avg_time );
+    
+        if (!isset($results[$filesize]))
+            $results[$filesize] = [];
+        $results[$filesize]['method3'] = $avg_time;
+    }
+
+}
+echo "Sending $receiver_count mails with only one PHPMailer instance and ReduceMIMEEncodeCacheStore: \n";
+{
+    foreach ($image_files as $filesize => $image_file) {
+        echo "\tWith $filesize attachment, path = $image_file\n";
+        $total_time = 0;
+        for ($i=1; $i<=$test_count_per_run ; ++$i) {
+            //echo "\t\tRun $i: ";
+            $start = microtime(true);
+            $cacheLookupTable = [];
+
+            $mail = new PHPMailer;
+            $mail->isSMTP();
+            $mail->SMTPDebug = 0;
+            $mail->Host = 'mail.example.com';
+            $mail->Port = 25;
+            $mail->SMTPAuth = true;
+            $mail->Username = 'yourname@example.com';
+            $mail->Password = 'yourpassword';
+            $mail->setFrom('from@example.com', 'First Last');
+            $mail->addReplyTo('replyto@example.com', 'First Last');
+
+            foreach ($receiver_list as $receiver) {
+                       $mail->clearAddresses();
+                $mail->addAddress($receiver);
+                $mail->Subject = 'Hi '.$receiver.'!';
+                $mail->msgHTML(str_replace( '<h1>This is a test of PHPMailer.</h1>', '<h1>Hi '.$receiver.'</h1>', file_get_contents('contents.html')), __DIR__);
+                $mail->AltBody = 'This is a plain-text message body for '.$receiver;
+    
+                //Attach an image file
+                $mail->addAttachment($image_file);
+
+                  $mail->ReduceMIMEEncodeCacheStore = &$cacheLookupTable;
+    
+                //Build the mail content
+                $mail->preSend(); 
+            }
+            $cost = microtime(true) - $start;
+            //echo "$cost sec\n";
+            $total_time += $cost;
+        }
+        $avg_time = $total_time / floatval($test_count_per_run);
+        echo sprintf("\t\ttest $test_count_per_run runs, avg. time: %.5f seconds\n", $avg_time );
+    
+        if (!isset($results[$filesize]))
+            $results[$filesize] = [];
+        $results[$filesize]['method4'] = $avg_time;
+    }
+}
+//
+// Print results
+echo "\n";
+echo "Size\tMethod1\tMethod2\tMethod3\tMethod4\n";
+foreach($results as $filesize => $test_method_result) {
+    echo "$filesize";
+    foreach($test_method_result as $value)
+        echo sprintf("\t%3.4f", $value);
+    echo "\n";
+}
diff --git a/examples/smtp_reduce_mime_encoding.phps b/examples/smtp_reduce_mime_encoding.phps
new file mode 100644
index 0000000000..f49aa48864
--- /dev/null
+++ b/examples/smtp_reduce_mime_encoding.phps
@@ -0,0 +1,232 @@
+<?php
+/**
+ * This example shows sending mail per receiver and reduce MIME encode.
+ */
+
+//Import the PHPMailer class into the global namespace
+use PHPMailer\PHPMailer\PHPMailer;
+
+//SMTP needs accurate times, and the PHP time zone MUST be set
+//This should be done in your php.ini, but this is how to do it if you don't have access to that
+date_default_timezone_set('Etc/UTC');
+
+require '../vendor/autoload.php';
+
+
+$receiver_count = 1000;
+
+$receiver_list = [];
+for ($i=0 ; $i<$receiver_count ; ++$i)
+    $receiver_list[] = 'whoto'.$i.'@example.com';
+
+echo "Genrate $receiver_count receivers: " . count($receiver_list) . "\n";
+
+echo "Sending $receiver_count mails with creating a new PHPMailer instance: ";
+
+$start = microtime(true);
+foreach ($receiver_list as $receiver) {
+
+    //Create a new PHPMailer instance
+    $mail = new PHPMailer;
+    //Tell PHPMailer to use SMTP
+    $mail->isSMTP();
+    //Enable SMTP debugging
+    // 0 = off (for production use)
+    // 1 = client messages
+    // 2 = client and server messages
+    $mail->SMTPDebug = 0;
+    //Set the hostname of the mail server
+    $mail->Host = 'mail.example.com';
+    //Set the SMTP port number - likely to be 25, 465 or 587
+    $mail->Port = 25;
+    //Whether to use SMTP authentication
+    $mail->SMTPAuth = true;
+    //Username to use for SMTP authentication
+    $mail->Username = 'yourname@example.com';
+    //Password to use for SMTP authentication
+    $mail->Password = 'yourpassword';
+    //Set who the message is to be sent from
+    $mail->setFrom('from@example.com', 'First Last');
+    //Set an alternative reply-to address
+    $mail->addReplyTo('replyto@example.com', 'First Last');
+    //Set who the message is to be sent to
+    $mail->addAddress($receiver);
+    //Set the subject line
+    $mail->Subject = 'Hi '.$receiver.'!';
+    //Read an HTML message body from an external file, convert referenced images to embedded,
+    //convert HTML into a basic plain-text alternative body
+    $mail->msgHTML(str_replace( '<h1>This is a test of PHPMailer.</h1>', '<h1>Hi '.$receiver.'</h1>', file_get_contents('contents.html')), __DIR__);
+    //Replace the plain text body with one created manually
+    $mail->AltBody = 'This is a plain-text message body for '.$receiver;
+    //Attach an image file
+    $mail->addAttachment('images/phpmailer_mini.png');
+
+    //Build the mail content
+    $mail->preSend(); 
+    unset($mail);
+}
+$cost = microtime(true) - $start;
+
+echo $cost . " sec\n";
+
+echo "Sending $receiver_count mails with only one PHPMailer instance: ";
+
+$start = microtime(true);
+{
+    //Create a new PHPMailer instance
+    $mail = new PHPMailer;
+    //Tell PHPMailer to use SMTP
+    $mail->isSMTP();
+    //Enable SMTP debugging
+    // 0 = off (for production use)
+    // 1 = client messages
+    // 2 = client and server messages
+    $mail->SMTPDebug = 0;
+    //Set the hostname of the mail server
+    $mail->Host = 'mail.example.com';
+    //Set the SMTP port number - likely to be 25, 465 or 587
+    $mail->Port = 25;
+    //Whether to use SMTP authentication
+    $mail->SMTPAuth = true;
+    //Username to use for SMTP authentication
+    $mail->Username = 'yourname@example.com';
+    //Password to use for SMTP authentication
+    $mail->Password = 'yourpassword';
+    //Set who the message is to be sent from
+    $mail->setFrom('from@example.com', 'First Last');
+    //Set an alternative reply-to address
+    $mail->addReplyTo('replyto@example.com', 'First Last');
+
+    //Attach an image file
+    $mail->addAttachment('images/phpmailer_mini.png');
+
+    foreach ($receiver_list as $receiver) {
+        //Reset addresses
+        $mail->clearAddresses();
+        //Set who the message is to be sent to
+        $mail->addAddress($receiver);
+        //Set the subject line
+        $mail->Subject = 'Hi '.$receiver.'!';
+        //Read an HTML message body from an external file, convert referenced images to embedded,
+        //convert HTML into a basic plain-text alternative body
+        $mail->msgHTML(str_replace( '<h1>This is a test of PHPMailer.</h1>', '<h1>Hi '.$receiver.'</h1>', file_get_contents('contents.html')), __DIR__);
+        //Replace the plain text body with one created manually
+        $mail->AltBody = 'This is a plain-text message body for '.$receiver;
+ 
+        //Build the mail content
+        $mail->preSend(); 
+    }
+}
+$cost = microtime(true) - $start;
+
+echo $cost . " sec\n";
+//
+// ---
+//
+echo "Sending $receiver_count mails with creating a new PHPMailer instance and ReduceMIMEEncodeCacheStore: ";
+
+$start = microtime(true);
+$cacheLookupTable = [];
+foreach ($receiver_list as $receiver) {
+    //Create a new PHPMailer instance
+    $mail = new PHPMailer;
+    //Tell PHPMailer to use SMTP
+    $mail->isSMTP();
+    //Enable SMTP debugging
+    // 0 = off (for production use)
+    // 1 = client messages
+    // 2 = client and server messages
+    $mail->SMTPDebug = 0;
+    //Set the hostname of the mail server
+    $mail->Host = 'mail.example.com';
+    //Set the SMTP port number - likely to be 25, 465 or 587
+    $mail->Port = 25;
+    //Whether to use SMTP authentication
+    $mail->SMTPAuth = true;
+    //Username to use for SMTP authentication
+    $mail->Username = 'yourname@example.com';
+    //Password to use for SMTP authentication
+    $mail->Password = 'yourpassword';
+    //Set who the message is to be sent from
+    $mail->setFrom('from@example.com', 'First Last');
+    //Set an alternative reply-to address
+    $mail->addReplyTo('replyto@example.com', 'First Last');
+    //Set who the message is to be sent to
+    $mail->addAddress($receiver);
+    //Set the subject line
+    $mail->Subject = 'Hi '.$receiver.'!';
+    //Read an HTML message body from an external file, convert referenced images to embedded,
+    //convert HTML into a basic plain-text alternative body
+    $mail->msgHTML(str_replace( '<h1>This is a test of PHPMailer.</h1>', '<h1>Hi '.$receiver.'</h1>', file_get_contents('contents.html')), __DIR__);
+    //Replace the plain text body with one created manually
+    $mail->AltBody = 'This is a plain-text message body for '.$receiver;
+    //Attach an image file
+    $mail->addAttachment('images/phpmailer_mini.png');
+
+    //Set Reduce MIME Encode Cache Store
+    $mail->ReduceMIMEEncodeCacheStore = &$cacheLookupTable;
+
+    //Build the mail content
+    $mail->preSend(); 
+    unset($mail);
+}
+$cost = microtime(true) - $start;
+
+echo $cost . " sec\n";
+//
+// ---
+//
+echo "Sending $receiver_count mails with only one PHPMailer instance and ReduceMIMEEncodeCacheStore: ";
+
+$start = microtime(true);
+$cacheLookupTable = [];
+{
+    //Create a new PHPMailer instance
+    $mail = new PHPMailer;
+    //Tell PHPMailer to use SMTP
+    $mail->isSMTP();
+    //Enable SMTP debugging
+    // 0 = off (for production use)
+    // 1 = client messages
+    // 2 = client and server messages
+    $mail->SMTPDebug = 0;
+    //Set the hostname of the mail server
+    $mail->Host = 'mail.example.com';
+    //Set the SMTP port number - likely to be 25, 465 or 587
+    $mail->Port = 25;
+    //Whether to use SMTP authentication
+    $mail->SMTPAuth = true;
+    //Username to use for SMTP authentication
+    $mail->Username = 'yourname@example.com';
+    //Password to use for SMTP authentication
+    $mail->Password = 'yourpassword';
+    //Set who the message is to be sent from
+    $mail->setFrom('from@example.com', 'First Last');
+    //Set an alternative reply-to address
+    $mail->addReplyTo('replyto@example.com', 'First Last');
+    //Attach an image file
+    $mail->addAttachment('images/phpmailer_mini.png');
+
+    //Set Reduce MIME Encode Cache Store
+    $mail->ReduceMIMEEncodeCacheStore = &$cacheLookupTable;
+
+    foreach ($receiver_list as $receiver) {
+        //Reset addresses
+        $mail->clearAddresses();
+        //Set who the message is to be sent to
+        $mail->addAddress($receiver);
+        //Set the subject line
+        $mail->Subject = 'Hi '.$receiver.'!';
+        //Read an HTML message body from an external file, convert referenced images to embedded,
+        //convert HTML into a basic plain-text alternative body
+        $mail->msgHTML(str_replace( '<h1>This is a test of PHPMailer.</h1>', '<h1>Hi '.$receiver.'</h1>', file_get_contents('contents.html')), __DIR__);
+        //Replace the plain text body with one created manually
+        $mail->AltBody = 'This is a plain-text message body for '.$receiver;
+ 
+        //Build the mail content
+        $mail->preSend(); 
+    }
+}
+$cost = microtime(true) - $start;
+
+echo $cost . " sec\n";
diff --git a/src/PHPMailer.php b/src/PHPMailer.php
index 16359fabe5..641082dfb2 100644
--- a/src/PHPMailer.php
+++ b/src/PHPMailer.php
@@ -420,6 +420,17 @@ class PHPMailer
      */
     protected $SingleToArray = [];
 
+    /**
+     * Whether to send the same message to each receiver
+     * with small changes in mail content.
+     * Use a lookup table to reduce attachment encoding.
+     *
+     * Storage for attachment encoding.
+     *
+     * @var array
+     */
+    public $ReduceMIMEEncodeCacheStore = false;
+
     /**
      * Whether to generate VERP addresses on send.
      * Only applicable when sending via SMTP.
@@ -3028,14 +3039,34 @@ protected function attachAll($disposition_type, $boundary)
     protected function encodeFile($path, $encoding = self::ENCODING_BASE64)
     {
         try {
-            if (!static::isPermittedPath($path) || !file_exists($path)) {
-                throw new Exception($this->lang('file_open') . $path, self::STOP_CONTINUE);
-            }
-            $file_buffer = file_get_contents($path);
-            if (false === $file_buffer) {
-                throw new Exception($this->lang('file_open') . $path, self::STOP_CONTINUE);
+            $file_buffer = '';
+            if ($this->ReduceMIMEEncodeCacheStore !== false) {
+                $cache_key = $path . "\t" . $encoding;
+                if (array_key_exists($cache_key, $this->ReduceMIMEEncodeCacheStore)) {
+                    $file_buffer = &$this->ReduceMIMEEncodeCacheStore[$cache_key];
+                    $this->edebug('encodeFile: use cache [' . $path . '][' . $encoding . ']');
+                } else {
+                    if (!static::isPermittedPath($path) || !file_exists($path)) {
+                        throw new Exception($this->lang('file_open') . $path, self::STOP_CONTINUE);
+                    }
+                    $file_buffer = file_get_contents($path);
+                    if (false === $file_buffer) {
+                        throw new Exception($this->lang('file_open') . $path, self::STOP_CONTINUE);
+                    }
+                    $file_buffer = $this->encodeString($file_buffer, $encoding);
+                    $this->ReduceMIMEEncodeCacheStore[$cache_key] = $file_buffer;
+                    $this->edebug('encodeFile: set cache [' . $path . '][' . $encoding . ']');
+                }
+            } else {
+                if (!static::isPermittedPath($path) || !file_exists($path)) {
+                    throw new Exception($this->lang('file_open') . $path, self::STOP_CONTINUE);
+                }
+                $file_buffer = file_get_contents($path);
+                if (false === $file_buffer) {
+                    throw new Exception($this->lang('file_open') . $path, self::STOP_CONTINUE);
+                }
+                $file_buffer = $this->encodeString($file_buffer, $encoding);
             }
-            $file_buffer = $this->encodeString($file_buffer, $encoding);
 
             return $file_buffer;
         } catch (Exception $exc) {