552 5.3.4: Message too big
552 5.3.4 means your message is larger than the receiving system accepts. The mailbox and the address are fine. The problem is the size of the email, which you control.
- Bounce type
- Hard bounce (permanent)
- Usually caused on
- Your sending side
- Terlido report label
- Unrecognised
What the standard says
RFC 5321 defines 552 as "exceeded storage allocation", which in practice covers messages that exceed a size limit. RFC 3463 defines 5.3.4 as "Message too big for system": the message is larger than the per-message size limit of the system. Subject 3 means the mail system, not the mailbox.
What the mailbox providers say
- Gmail (552 5.3.4): “Your message exceeded Google's message size limits...” Source
Common causes
- Large attachments, such as a PDF deck, a video, or several images.
- Heavy images embedded in the body at full resolution.
- Very large HTML, for example from a template exported from a design tool with a lot of inline code.
- Too many attachments or very large headers. Google also lists limits on attachment count and header size.
- A signature that includes a large banner image or many small images, which adds up to a large message with every send.
- A forwarded or quoted thread pasted into the body, which can add a lot of hidden content to the message size.
How to fix it
- Remove the attachment and link to the file instead. Cold email works better without attachments anyway.
- Resize and compress images before you add them, and use fewer of them.
- Strip extra markup from the HTML, or send a plain text version.
- Check the full size of the message, including images and headers, before you launch the sequence.
- Replace image-heavy signatures with plain text, and host any large image on your own site.
- Send a short test to a Gmail address and look at the size of the message in the headers before you launch.
- Keep the first email in a sequence short and free of images, which also makes it feel more personal.
Is it a deliverability problem?
This is about your message and not about reputation. Smaller emails are also easier on receiving systems, so the fix is cheap and has no downside. Your bounce report shows this code as Unrecognised on purpose, because blaming the recipient's mailbox would be wrong. Because size limits are the same for every send, one oversized template can fail an entire campaign in a few minutes. Test once before you launch, since fixing the template afterwards cannot bring back the contacts who already received a failure and may have stopped the sequence.
How Terlido handles it
When a bounce message for this code arrives in a connected mailbox, Terlido reads the status code. An enhanced code that starts with 5 is a hard bounce. In the campaign Reports tab this one is grouped as "Unrecognised".
On a hard bounce Terlido adds the address to the workspace blocklist with the reason "hard bounce", so no campaign emails it again. The lead stops in that campaign as bounced, its lead score goes down by 15, and the bounce counts toward the campaign's bounce rate.
Once a campaign has at least 20 sends, it pauses itself when hard bounces divided by sends goes over the campaign's maximum bounce rate. The default is 5%. If the cause was on your side and you fixed it, you can remove the entry on the Blocklist page of your account settings and try again.
Check the domain behind the bounce
The free domain checker runs live SPF, DKIM, DMARC, MX and blocklist lookups and explains each result in plain English.
Free tools that help
- Email size checkerMeasure raw HTML and text byte size.
- Image resizer & converterResize an image and export JPEG, PNG or WebP in your browser. Choose dimensions and quality without uploading your file.
- HTML email to plain textConvert HTML to readable text with link destinations.