Packaging SMTP(send only) APIs to a choosable option is the goal here, then to have a tiny server that picks 1 provider and serves as HTTP endpoint so the other microservices that belong to you can make email requests.

• Brevo (highly recommended)
• Resend
• Postmark
• Mailchimp
• Mailtrap
• Mailjet
• Mailgun
• Sendgrid (avoid them if you can)

Note: Anything that only uses oauth2 like zoho does will not be implemented here for foreseeable future