What the message looks like
552 5.3.4 Error: message file too big552 5.3.4 Message size exceeds fixed limit552-5.3.4 Your message exceeded Google's message size limits550 5.2.3 RESOLVER.RST.RecipSizeLimit; message too large for this recipient
What it means
The email is bigger than the receiving server will accept. The standard name for this status is "message too big for system".
It applies to that one email, not to the address, so smaller emails to the same person still get through. Some servers use 5.2.3 instead, meaning the email is bigger than the limit for that particular mailbox.
Gmail also uses 552 5.3.4 when a single header line is too long, which is usually a problem with the software that created the email rather than its size.
Why emails end up bigger than expected
Attachments are encoded before they're sent, which makes them about a third larger. A 20MB file becomes roughly 27MB once it's in an email, so it can bounce even when the file itself is under the limit. Images pasted into the body count too.
Most providers accept emails of around 20 to 35MB in total, but some company servers set much lower limits.
If you're the recipient
Ask the sender to share the file with a link instead, or to send it in smaller parts. If you need to receive large attachments regularly, check your provider's size limit.
If you're the sender
Upload the file to a file sharing or cloud storage service and send a link, compress it, or split it across several emails. Remember the encoded size is larger than the file size.
If you use addy.io
addy.io accepts emails up to 25MB, including attachments. Larger emails are rejected with 552 5.3.4 Error: message file too big before they reach your alias. Your own inbox provider may have a lower limit, in which case the forward appears on your Failed Deliveries page.