SendGrid 552 message too large or mailbox full

Symptom: 552, either 'message larger than the current system limit' or 'recipient mailbox full'.

1. If the message is too large: shorten the body and drop or link attachments instead of embedding them. Then resend.
2. If the mailbox is full: do not resend on a loop. One retry after a day is reasonable; a full mailbox is usually an abandoned address.
3. Tell the two cases apart from the human-readable portion of the 552 text before deciding.
4. Track 552s per recipient. Repeated mailbox-full on the same address is a dead address wearing a temporary error's clothes: suppress it.
5. The v3 API equivalent is 413 content too large. Same root cause, same fix.

552 is permanent for that attempt. It is not a deferral and will not be retried for you.