What 552 5.2.3 Means
The receiving server can't accept your message because it exceeds the per-message size limit. Per Gmail's documented limits, the max is 25 MB for messages received at consumer Gmail. Google Workspace admins can configure higher limits per-tenant, but 25 MB is the typical ceiling.
Gmail consumer accounts and Google Workspace domains. Most other receivers have similar limits (Microsoft 365 defaults to 25 MB, Yahoo around 25 MB).
Your message body + attachments + embedded images + MIME overhead exceed 25 MB. Note that attachments are base64-encoded in SMTP, which adds ~33% overhead — a 20 MB raw file becomes ~27 MB encoded, tipping over the limit.
How to Fix 552 5.2.3
- 1
Calculate your actual SMTP message size
The SMTP size is the encoded size (base64 for binary attachments), not the raw file size. Multiply your raw attachment size by ~1.37 to estimate. If your attachments total >18 MB raw, you're at risk of exceeding the 25 MB encoded limit.
- 2
Strip attachments and use links instead
Cold email essentially never benefits from large attachments. Host the file on Drive, Dropbox, S3, or your own server, and embed a link. Receivers prefer links to attachments, and you bypass the size limit entirely.
- 3
Compress images if you must embed them
Embedded images often inflate cold email size unnecessarily. Optimize PNG/JPEG to web-quality (typically 100-300 KB per image is plenty). Modern formats (WebP) compress better but aren't supported in all clients.
- 4
Avoid sending images at all
Image-heavy cold email looks like marketing newsletter rather than personal outreach. Plain text or minimal HTML cold emails have better deliverability AND smaller size. The rare case where embedded images help is signature personalization — keep these small.
- 5
Check your tracking pixel and footer
Some sending platforms inject substantial HTML for tracking pixels, unsubscribe links, and analytics. If your message size is unexpectedly large with minimal content, audit your platform's default injected HTML. Some let you opt out of full tracking suites.
References
552 5.2.3 in the Cold Email Context
Message size limits rarely affect cold email because cold email is typically text or simple HTML — well under 100 KB. The cases where 5.2.3 does appear: salespeople attaching a PDF deck (often 5-10 MB) or sending an embedded video preview (large). The deliverability-safe pattern is text + link to anything heavier. Cloud hosting for shared assets is cheap; deliverability cost of attachment-heavy mail is high. ColdRelay's infrastructure doesn't change message-size dynamics — but the cold-email best-practices ColdRelay's deliverability consulting recommends (text-only or minimal-HTML, no attachments) naturally avoid this error.
Frequently Asked Questions
What's the exact Gmail size limit?
25 MB for inbound to consumer Gmail. Google Workspace admins can set higher inbound limits up to 50 MB. Outbound sends from Gmail are capped at 25 MB by Google's outbound infrastructure regardless of recipient.
Why is the encoded size larger than the raw file?
SMTP only supports 7-bit ASCII. Binary attachments are base64-encoded, which inflates 3 bytes of binary to 4 bytes of ASCII — a 33% size increase. Plain-text or already-ASCII content doesn't grow.
Does using SMTP SIZE extension help?
It lets the receiver reject early (before you transmit the full message body), which saves bandwidth but doesn't bypass the limit. The receiver returns the same 552 5.2.3 after you announce size.
Can I split a large attachment across multiple messages?
Technically yes, but it's a terrible cold-email pattern. Most recipients won't reassemble multi-part attachments. Use a link to a single hosted file instead — that's what cloud storage exists for.