A simple mail server written in Java

Client sends a HELO command to the server

• C: HELO example.com
• S: 250 Hello example.com, pleased to meet you.

Client sends a MAIL FROM command to the server

• C: MAIL FROM:[email protected]
• S: 250 Ok

Client sends a RCPT TO command to the server

• C: RCPT TO:[email protected]
• S: 250 Ok

Client sends a DATA command to the server

• C: DATA
• S: 354 Enter message, ending with "end of email" on a line by itself

Client sends the email message to the server

• C: Subject: This is the subject of the email
• C:
• C: This is the body of the email
• C: end of email
• S: 250 Ok: queued as 12345

Client sends a QUIT command to the server

• C: QUIT
• S: 221 Bye

-- In the above simulation, the client initiates the interaction by sending a 'HELO' command to the server to introduce itself. The server responds with a greeting and a message indicating that it's pleased to meet the client.

-- Next, the client sends a 'MAIL FROM' command to identify the sender of the email message. The server responds with an acknowledgement that the sender has been identified.

-- The client then sends a 'RCPT TO' command to identify the recipient of the email message. The server again responds with an acknowledgement that the recipient has been identified.

-- The client then sends a 'DATA' command to indicate that it will be sending the actual email message to the server. The server responds with a message indicating that it's ready to receive the email message.

-- The client then sends the email message to the server, including the subject and body of the message. After the client is finished sending the message, it sends a period (".") on a line by itself to indicate the end of the message. The server responds with an acknowledgement that the message has been queued for delivery.

-- Finally, the client sends a 'QUIT' command to indicate that it's finished with the SMTP session. The server responds with a goodbye message and closes the connection.

For more info you can visit : https://postmarkapp.com/guides/everything-you-need-to-know-about-smtp