What is the minimum message size a server must accept per RFC 5321?

The minimum message size a server must accept per RFC 5321 is 64 kilobytes.

The standard expresses this as 64K octets, covering the transmitted message content rather than the SMTP command exchange itself. This requirement prevents a receiving server from imposing an excessively small universal limit on ordinary email. Servers may accept much larger messages, and real-world providers commonly impose their own limits for storage, abuse prevention, and bandwidth reasons.

A common mix-up is confusing this rule with other SMTP limits. RFC 5321 also limits an SMTP command line to 512 characters, while the maximum length of a text line in message data is 1000 characters including the terminating carriage-return and line-feed. Those are line-length rules, not the minimum total message size.

The 64K requirement originated in earlier SMTP specifications, including RFC 2821, and was carried into RFC 5321. It does not mean every email is limited to 64 kilobytes; it means a compliant server cannot reject a message solely because it is below that minimum capacity threshold.