What’s the difference between SMTP, MIME, and ESMTP?

SMTP (Simple Mail Transfer Protocol) is the core protocol for sending email between servers. Defined in RFC 5321, it handles the actual transmission of messages from sender to recipient servers through command and response exchanges.

ESMTP (Extended SMTP) adds capabilities to basic SMTP through extensions negotiated using the EHLO command. Extensions enable features like authentication (SMTP AUTH), encryption (STARTTLS), and size limits. Modern email servers use ESMTP.

MIME (Multipurpose Internet Mail Extensions) extends the basic text format to support attachments, HTML content, character sets, and multi part messages. MIME works on top of SMTP/ESMTP, encoding complex content for transmission through the text based protocol.