My attachment is under the limit, so why 552?

Base64 encoding adds about 33 percent, and the body and other attachments count too, so the message on the wire is larger than the file.

Will the message deliver once the mailbox has space?

Not by itself. 552 is permanent, so the sender has to send it again after the recipient clears space.

SMTP 552: exceeded storage allocation or size

552Requested mail action aborted: exceeded storage allocationPermanent: retrying will not help

Example

552 5.3.4 Message size exceeds fixed limit

What it means

RFC 5321 defines 552 as "Requested mail action aborted: exceeded storage allocation". It is permanent, and in practice means one of two things: the message is larger than the receiver accepts, or the recipient’s mailbox has no room.

With 5.3.4, "Message too big for system", the server’s size limit is the issue. Remember that attachments grow by about a third when Base64 encoded, so a 20 MB file becomes roughly 27 MB on the wire. With 5.2.2, "Mailbox full", the recipient is over quota, and Gmail says "The email account that you tried to reach is over quota". 5.2.3 is a per mailbox message size limit.

RFC 5321 also records that some old servers used 552 for too many recipients; the correct reply for that is now 452. Use 452 as the temporary comparison: same storage idea, but retryable.

Likely causes

The message, after encoding, is bigger than the receiver’s maximum size (often 25 MB at Gmail and 35 to 150 MB at Microsoft 365 depending on settings).
The recipient’s mailbox is full and their provider rejects new mail permanently instead of deferring.
A per user or per domain size rule on the recipient’s side, such as a transport rule in Exchange.
An old server answers 552 for too many recipients in one message.

How to fix it

Check the enhanced code: 5.3.4 or 5.2.3 means shrink the message, 5.2.2 means the recipient must clear space.
Share large files by link from a file sharing service instead of attaching them, or compress them.
Look at the SIZE value the receiver advertises after EHLO and keep the encoded message below it.
For a full mailbox, contact the recipient by another channel and resend once they have freed space.

Checks that apply

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

SMTP Diagnostics Shows the SIZE limit in the receiving server’s EHLO reply, so you know the exact maximum before you resend.
MX Lookup Identifies the recipient’s mail provider from its MX hosts, which tells you which published size limits apply.

FAQ

My attachment is under the limit, so why 552?: Base64 encoding adds about 33 percent, and the body and other attachments count too, so the message on the wire is larger than the file.
Will the message deliver once the mailbox has space?: Not by itself. 552 is permanent, so the sender has to send it again after the recipient clears space.

All 191 bounce codes