What does “message too large” mean?

You sent an email, it bounced back, and the error says something like "552 Message size exceeds limit" or "5.3.4 Message too big." That's a message-too-large bounce. It means the receiving server looked at your email, decided it was over their size limit, and rejected it before it ever hit the inbox.

The most common culprit is attachments. A single PDF, a slide deck, or a few high-res photos can push you over the limit fast. But attachments aren't the only thing adding weight. Embedded images in your HTML, heavy inline CSS, and the way email encodes files (a process called base64) all inflate the final size. Base64 encoding alone adds roughly 33% on top of the raw file size, so a 10MB attachment becomes closer to 13MB by the time it leaves your outbox.

As for the limits themselves, Gmail and Outlook both accept messages up to 25MB. Corporate mail servers are often stricter, with limits at 10MB or even lower. Some individual mailboxes also have their own caps set by the admin, which can be tighter than the server-wide default.

The fix is usually one of three things. First, swap attachments for cloud links. Upload the file to Google Drive or Dropbox and paste the link in the email body instead. Recipients navigate to it themselves, which is also safer for them (security gateways are suspicious of emails with download links baked in). Second, compress your files before attaching. A zipped folder or a compressed PDF can shrink the size significantly. Third, if you're sending a lot of inline images in an HTML email, host them externally and reference them by URL rather than embedding them directly.

If you're not sure what's actually bloating your message, run it through our free Email Header Analyzer to see what the SMTP bounce code is telling you. Or if you're getting these on bulk sends and can't figure out what's causing it, the SOS hotline is free and we'll look at it with you.