SendGrid 413 content too large: attachments blew the request size

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.

The errors doc lists 413 as content too large. The mail send reference documents attachments as base64 content with type, filename, and disposition. The SMTP errors doc lists 552 for messages larger than the system limit. Sources: https://docs.sendgrid.com/api-reference/how-to-use-the-sendgrid-v3-api/errors | https://docs.sendgrid.com/api-reference/mail-send/mail-send | http://sendgrid.com/docs/Glossary/emailerrormessages.html