What is the biggest attachment I can send?

It depends on the receiver. Divide the advertised SIZE by about 1.37 to allow for encoding and headers.

Was the message partly delivered?

No. An oversized message is refused whole; nobody received it.

5.3.4 bounce: message too big for system

5.3.4Message too big for systemPermanent: retrying will not help

Example

552 5.3.4 Message size exceeds fixed limit

What it means

X.3.4 is "Message too big for system" in RFC 3463. The message exceeds the largest size the receiving mail system will accept from anyone. The example is Postfix’s exact wording when message_size_limit is exceeded; RFC 5248 pairs the code with basic replies 552 and 554.

Well-behaved servers announce their limit in the SIZE parameter of their EHLO reply (RFC 1870), so your server can refuse to send an oversized message before transmitting it. In that case your own MTA produces the 5.3.4 bounce without contacting the recipient. Typical limits are 25 MB at Gmail and Yahoo, 35 MB for Exchange Online by default (configurable up to 150 MB), and 10 MB for an untouched Postfix install.

Compare 5.2.3, which is a per-mailbox or policy limit, and 5.2.2, which is a full mailbox. A temporary 4.3.4 is rare; size limits do not change between retries.

Likely causes

An attachment that is too large once base64 encoding adds about 33 percent.
A long reply chain carrying several earlier attachments plus inline images.
The recipient’s server uses a default limit that is lower than yours (a stock Postfix 10 MB limit, for instance).
A relay or anti-spam gateway in the path has a smaller limit than the final server.

How to fix it

Send large files as a link to cloud storage instead of attaching them.
If you must attach, compress or split the files so each message stays well under the recipient’s SIZE limit.
Check the recipient server’s advertised limit before resending, since it may be far lower than your own.
If you run the receiving server and need larger mail, raise the limit, for example `message_size_limit = 52428800` in Postfix, and the matching limit on any gateway in front of it.

Checks that apply

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

SMTP Diagnostics Reads the SIZE value the recipient server advertises in EHLO, which is the real ceiling for your message.
MX Lookup Shows all MX hosts, since a gateway with a smaller limit may sit in front of the final server.

FAQ

What is the biggest attachment I can send?: It depends on the receiver. Divide the advertised SIZE by about 1.37 to allow for encoding and headers.
Was the message partly delivered?: No. An oversized message is refused whole; nobody received it.

All 191 bounce codes