Symptom: 413 content too large on /mail/send, usually right after adding attachments.
1. Remember base64 inflates files by about a third. A 20 MB file is ~27 MB on the wire before the rest of the JSON.
2. Check each attachment entry has content (base64), type, filename. A missing type or a double-encoded file bloats or breaks the payload.
3. Fix: host large files and link them instead of attaching. Attachments are for small files the recipient must have offline.
4. Compress or downscale before attaching: PDFs, images, and decks usually shrink a lot.
5. If you must send big, split across messages rather than one giant request.
Over SMTP the same problem surfaces as 552 (message larger than the system limit). Same fix.