Does a 452 mean my message was lost?

No. The refused recipients stay in your queue and your server retries them. Only after the queue lifetime ends does a bounce go back to you.

What is the difference between 452 and 552?

452 is temporary (storage short now, or too many recipients this time). 552 is permanent, typically a message over the size limit or a hard quota.

SMTP 452: insufficient storage or too many recipients

452Requested action not taken: insufficient system storageTemporary: the sender retries

Example

452 4.5.3 Error: too many recipients

What it means

RFC 5321 names 452 "Requested action not taken: insufficient system storage". It is temporary, so the sending server is expected to try again. In real traffic it covers two quite different situations, and the enhanced code tells you which one you have.

With 4.3.1 or 4.2.2 the receiving system or the recipient’s mailbox is short of space. With 4.5.3 the server is refusing extra RCPT TO commands because the message already has as many recipients as it will take in one transaction. RFC 5321 section 4.5.3.1.10 says servers should use 452 for that limit, and that clients should send the remaining recipients in a later transaction.

Compare it with 552, its permanent neighbour. 552 is used for messages that exceed a size limit or a mailbox quota for good, while 452 means "not now, try again or split it up".

Likely causes

Your message lists more recipients than the receiver accepts per transaction (Postfix defaults to 1000; many providers use 100).
The recipient’s mailbox is at or near its quota and the server defers new mail (452 4.2.2).
The receiving server’s spool disk is full or below its free space threshold (452 4.3.1).
Your own MTA or application sends all recipients in one session without honouring 452 replies.

How to fix it

Check the enhanced code. For 4.5.3, configure your sender to batch recipients (for example 50 per message) and send the rest in a new transaction.
Make sure your MTA treats 452 on RCPT as "retry these addresses later". Postfix and Exim do this already; some custom scripts do not.
For 4.2.2, tell the recipient that their mailbox is full; the message will deliver once they clear space, as long as your queue has not expired.
If you run the receiving server and it reports 4.3.1, free disk space on the spool volume and check queue size alerts.

Checks that apply

Your domain or sending IP (fills in the checks below)

SMTP Diagnostics Shows the receiving server’s banner and EHLO extensions, including the SIZE limit, so you can see its limits before you resend.
MailFlow Monitor Sends test mail through your path and watches it land, which confirms a batching change actually fixed the deferrals.

FAQ

Does a 452 mean my message was lost?: No. The refused recipients stay in your queue and your server retries them. Only after the queue lifetime ends does a bounce go back to you.
What is the difference between 452 and 552?: 452 is temporary (storage short now, or too many recipients this time). 552 is permanent, typically a message over the size limit or a hard quota.

All 191 bounce codes