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552 5.3.4: Message too big

Your message (or its headers, or attachment count) is over the receiver's size limit.

Bounce type
Hard bounce (permanent)
What to do
Fix your setup, keep the contact
Returned by
Gmail / Google Workspace; Any mail server

What 552 5.3.4 means

5.3.4 is 'Message too big for system': the message is larger than the receiver's per-message limit. RFC 3463 says it's only useful as a permanent error, so resending the same message will fail again.

Gmail has several 552 5.3.4 texts: overall message size, number of attachments, total header size, a single header that's too long, and an over-long Subject line. The text tells you which limit you hit.

What the bounce looks like

The wording providers document for this code. Your bounce may add an IP address, a session ID or a help link after it.

Gmail
552 5.3.4 Your message exceeded Google's message size limits.
Gmail
552 5.3.4 Your message has a Subject: header that exceeds Google's message header size limits.

Why cold emailers see 552 5.3.4

  • Attachments, especially PDFs or decks, in a cold email.
  • Large inline images or base64-embedded images in HTML templates.
  • Headers bloated by a tool that adds long custom headers, or a very long Subject.

How to fix 552 5.3.4

  1. Remove attachments from cold emails entirely. Offer the deck and send it when someone replies.
  2. Host images instead of embedding them, or better, send plain text.
  3. Check the raw message for oversized headers and shorten the subject line.
  4. Resend the reduced message to the same recipient.

Retry or suppress?

Don't suppress the recipient. Fix the message and resend. The address is valid.

Reading the code

The basic code 552 means "Exceeded storage allocation / message too big (permanent)" under RFC 5321. In 5.3.4, the 5 is the class (permanent failure) and the 3 is the subject (mail system). The detail number 4 is registered in RFC 3463 / the IANA registry as "Message too big for system".

Class 5
Permanent failure
Resending the same message won't work. Something about the message, the recipient or the sender has to change first.
Subject 3
Mail system
A problem with the destination mail system, such as size limits or configuration.
Detail 5.3.4
Message too big for system

Check the rest of your setup

Paste the headers of a message that did get through into the email header analyzer to confirm SPF, DKIM and DMARC pass, and check your domain with the SPF, DKIM & DMARC checker. The deliverability checklist covers the rest of a healthy sending setup.

Got a different bounce? Paste it into the SMTP error code lookup to decode it.

FAQ

Is 552 5.3.4 a hard bounce or a soft bounce?

552 5.3.4 is a permanent failure (a hard bounce): the receiving server won't accept this message if you send it again unchanged. But the cause is not the recipient's address, so treat it differently from a 'user unknown' bounce.

Should I remove the contact after a 552 5.3.4 bounce?

No. Don't suppress the recipient. Fix the message and resend. The address is valid.

What does 5.3.4 mean in the error code?

In 5.3.4, the 5 is the class (permanent failure) and the 3 is the subject (mail system). The detail number 4 is registered in RFC 3463 / the IANA registry as "Message too big for system".

Related SMTP codes

See all SMTP error codes

Sources

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Clement reads the bounces that land in your Gmail and SMTP inboxes and tells dead addresses apart from policy blocks: it suppresses the first across every campaign and keeps the contact on the second, so you don't have to decode bounces by hand.

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