We had a user create a report and scheduling it for delivery but the email was never received.
After a bunch of digging I found that the report resulted in an 80M PDF attachment that was, of course, undeliverable.
The only indication is the vague failure message in the sugar log which does not indicate what actually failed to send.
[FATAL] PHPMailerProxy encountered an error: SMTP Error: data not accepted.SMTP server error: DATA END command...