Mailpro’s SMTP relay accepts messages up to 20 MB total, including the email body, headers and all attachments after Base64 encoding (which adds about 33% to the size of binary files). For most transactional and marketing emails this is far more than enough — but a 15 MB attached PDF will end up close to 20 MB once encoded.
What counts toward the SMTP message limit
The 20 MB cap is the size of the entire RFC 5322 message after MIME encoding, not just the visible body. The breakdown is:
| Component | Typical size | Notes |
|---|---|---|
| HTML body | 20–100 KB | Compress and minify |
| Plain-text alternative | 5–20 KB | Required for spam filters |
| Inline images (CID) | 100 KB–5 MB | Each adds ~33% after Base64 |
| File attachments | Up to remaining quota | PDF, DOCX, ZIP |
| MIME overhead + headers | ~5 KB | Boundary markers, From, DKIM signature |
Why mailbox providers also impose limits
Even if Mailpro accepts 20 MB, the receiving server may reject anything above its own ceiling. Common limits in 2026:
| Provider | Max incoming size |
|---|---|
| Gmail | 25 MB |
| Outlook / Microsoft 365 | 20 MB (default), up to 150 MB if configured |
| Yahoo | 25 MB |
| Apple iCloud | 20 MB |
| Most enterprise gateways | 10–25 MB |
What to do with large attachments
1. Compress images (TinyPNG, Squoosh) before attaching. 2. Convert PDF/DOCX to PDF/A or compressed PDF (Acrobat «Reduce file size»). 3. Use a download link to a hosted file instead of an attachment. 4. For multiple files, send a single ZIP. 5. For very large files, use a transfer service (WeTransfer, Google Drive) and link.
Limits on the marketing campaign side
For broadcast campaigns built in the Mailpro Builder, the recommended HTML weight is under 100 KB to avoid Gmail’s message clipping. See file sizes and extensions for the per-file limits in the Builder and the SMTP relay FAQ for details on the SMTP path.
Send big emails the right way
For files larger than 5 MB, host them and link from the email. Mailpro’s SMTP relay and Email API both accept up to 20 MB but smaller payloads always deliver faster and into more inboxes.