Recipient limit
The most recipients a server accepts in one SMTP transaction; RFC 5321 requires at least 100 and gives 452 as the reply for going over a server's own limit.
What it means
Every server must accept at least 100 RCPT commands per message, and a client with more recipients should be ready to send them in batches of 100. A server that reaches its limit should answer further recipients with 452, too many recipients, while keeping the ones already accepted; the client delivers to those and sends the rest in a new transaction. RFC 5321 notes that RFC 821 wrongly listed 552 for this case and tells clients to treat a 552 here as temporary.
Providers add their own limits per transaction, per day and per account. Google's SMTP relay service accepts at most 100 recipients per transaction, Gmail answers too many recipients with 452 4.5.3, and Xfinity documents 452 replies for too many recipients in a message.
Related terms
On this site
Sources
What this definition rests on. The wording is ours; the facts are theirs.
- RFC 5321, section 4.5.3.1.8: Recipients Bufferrfc-editor.org
- RFC 5321, section 4.5.3.1.10: Too Many Recipients Coderfc-editor.org
- Google Workspace Admin Help: Route outgoing SMTP relay messages through Googleknowledge.workspace.google.com
- Google Workspace Admin Help: Gmail SMTP errors and codesknowledge.workspace.google.com
- Xfinity Support: Email error codesxfinity.com
All 432 terms in the glossaryThe wiki: blocklists and bounce codes