SIZE extension
An SMTP extension (RFC 1870) through which a server announces its maximum message size and a client declares the size of a message before sending it.
What it means
A server that supports it lists SIZE in its EHLO reply, usually with a number: the largest message it accepts, in octets (SIZE 52428800 is 50 MiB). The client adds SIZE= with its estimate to MAIL FROM, and the server can refuse at once instead of after the whole message has been transferred: 552 when the message exceeds the fixed limit, 452 when it is short of space for now.
The size that counts is the encoded size. Base64 turns every 3 bytes into 4 characters, so an attachment grows by about a third in transit, and a file that looks small enough on disk can still exceed a limit. Gmail refuses messages over its limit with 552 5.3.4.
Related terms
On this site
Sources
What this definition rests on. The wording is ours; the facts are theirs.
- RFC 1870: SMTP Service Extension for Message Size Declarationrfc-editor.org
- RFC 2045, section 6.8: Base64 Content-Transfer-Encodingrfc-editor.org
- Google Workspace Admin Help: Gmail SMTP errors and codesknowledge.workspace.google.com
All 432 terms in the glossaryThe wiki: blocklists and bounce codes