What 552 5.3.4 Means
Per RFC 1870 (SMTP Size Extension) and RFC 5321, receivers advertise a maximum message size in the EHLO response. If your message exceeds that limit, the receiver rejects with 552 5.3.4 'message size exceeds fixed limit'. The receiver's limit is typically 10 MB (consumer Gmail's traditional cap), 25-35 MB (most enterprise gateways including Microsoft 365), or higher for specially-configured tenants.
All major receivers. Gmail's consumer attachment limit is 25 MB; Workspace can go higher for paid plans. Microsoft 365's default is 35 MB. Some receivers enforce stricter limits via transport rules.
Attachment(s) total exceed the receiver's policy; embedded images in HTML body push total over the limit; signature contains heavy embedded graphics; the message body itself was constructed from a large text dump without compression.
How to Fix 552 5.3.4
- 1
Measure the actual message size
Total message size includes envelope, headers, body, and base64-encoded attachments. Base64 encoding adds ~33% overhead, so a 20 MB PDF becomes ~27 MB on the wire. Use your sending platform's preview to see the actual transmission size — it's almost always larger than just the file size.
- 2
Replace attachments with links
For cold email specifically, attachments are almost never appropriate — they trigger spam filters, get blocked by enterprise gateways, and don't track engagement. Upload the file to a hosted location (Drive, Dropbox, or a content URL on your own domain) and link to it in the body. Bonus: you can track clicks.
- 3
Strip embedded images from signature
Many email signatures embed company logos, profile photos, and social-icon images directly. Each embedded image counts toward total size. Use linked-image references (URLs pointing to public-hosted images) instead of inline base64. For cold email, consider a text-only signature — it's faster to load and harder to flag as marketing.
- 4
Check the receiver's published size limit
Some enterprise gateways advertise a tighter limit than the receiver's tenant policy actually enforces. The EHLO response from the receiver's MX includes the size declaration. If your platform exposes raw SMTP transcript, check what the receiver actually accepts.
- 5
Split the message if essential
If you absolutely must send a large file (not a cold email use case, more for transactional/document delivery), split into multiple smaller emails. ZIP compression can reduce text-heavy attachments but doesn't help PDFs or images that are already compressed.
References
552 5.3.4 in the Cold Email Context
552 5.3.4 in cold email is almost always self-inflicted: someone tried to attach a case study PDF, a proposal deck, or a product brochure. Don't. Cold email attachments don't get opened — they get blocked by enterprise gateways or treated as suspicious by spam filters. The conversion-rate-optimized pattern is a one-sentence pitch with a link to a hosted asset (with the link domain matching your sending domain so it doesn't look like a third-party tracker). ColdRelay's Sends log surfaces 5.3.4 bounces in the rejected-messages view; if you see any, your sending workflow has an attachment problem to fix at the template layer.
Frequently Asked Questions
What's Gmail's actual attachment size limit?
25 MB for sending (the SMTP-layer limit) — but the displayed limit in the Gmail web UI is 25 MB because Gmail itself transforms anything over the limit to a Drive link. For inbound, Gmail accepts up to 25 MB at the SMTP layer; larger messages bounce with 552.
Does Microsoft 365 enforce 35 MB strictly?
The default is 35 MB but tenant admins can configure both lower and higher limits. Enterprise tenants sometimes lower to 10 MB for inbound from external senders. The receiver's policy is what counts — your sending policy is irrelevant.
Is 552 5.3.4 the same as 552 5.2.3?
Closely related. 5.3.4 is 'message size limit exceeded' at the receiver's overall policy level. 5.2.3 is 'message length exceeds administrative limit' for a specific recipient mailbox. The remediation is the same; the difference is whether the limit is server-wide or per-recipient.
Why does base64 encoding inflate size?
SMTP transmits 7-bit ASCII traditionally. Binary attachments (PDFs, images) get base64-encoded, which uses 4 ASCII characters to represent every 3 binary bytes — a 33% overhead. A 30 MB binary attachment becomes ~40 MB on the wire.