Simple Mail Transfer Protocol

lastmod 2025-07-05created 2018-07-3118 edits

Simple Mail Transfer Protocol (SMTP) is a protocol used to send and receive email on the Internet. It uses TCP port 25. The MX record of DNS is used to designate the destination server. It is specified in RFC 2821. Besides transfer between mail servers, it is also often used when a mail client sends mail to a mail server.

SMTP is a text-based protocol, and it specifies that not only request/response messages but all characters must be 7-bit ASCII. Because of this, languages that use codes of 8 bits or more to represent characters, and the various binaries frequently used as attachments, are converted to 7 bits using a method called MIME before being transmitted.

SMTP does not specify how messages are created. Local editing or a simple email application is used to create messages. Once a message is created, the invoked SMTP receives it and passes it to the SMTP of another host using TCP.

Categories

• MIME
• Sender Policy Framework (SPF)
• maddy - all-in-one mail server
• Internet Message Access Protocol (IMAP)

Tools

• imapapi - access IMAP/SMTP accounts via REST.
• Trusted Messaging Transfer Protocol (TMTP)
  • mnm - a project building a client, server, and protocol (TMTP) all together to replace email
• postfix
  • PostfixAdmin - web based administration interface for Postfix mail servers
• sendmail
• MailHog

Libraries

• Python:smtplib

Cloud Service

• 10 Best Free SMTP Servers for Transactional Emails in 2024
• 11 Best Free SMTP Servers 2025: Up to 12k emails/month for free
• SMTP.com
• Moosend SMTP
• Amazon SES
• MailerSend - free plan: up to 3,000 emails per month
• SMTP2Go - free plan: up to 1,000 emails per month
• SendPulse - free plan: up to 12,000 emails per month
• Brevo (formerly Sendinblue) - free plan: up to 300 emails per day
• Mailjet - free plan: up to 200 emails per day
• Aha Send - free plan: up to 1,000 emails per month
• Mailtrap - free plan: up to 1,000 emails per month
• Postmark - free plan: up to 100 emails per month
• SendGrid - free plan: up to 100 emails per month
• Gmail - free plan: up to 500 emails per day
• Elastic Email - free plan: up to 100 emails per day

Terms

• VRFY: a command an SMTP client sends to an SMTP server to verify whether mail exists for a specific ID
• EXPN (expand mailing list): a command for forwarding when sending mail

Favorite site

• Wikipedia (en) description of SMTP
• CodeProject SMTP sending sample ¹

References