We ran into an issue where our proofs were not being delivered because of invalid characters in the headers. The error in Campaign was this:
Not Defined" as Nature of Failure, and Reason "550 5.6.0 Lone CR or LF in headers (see RFC2822 section 2.2)
This ended up being pretty difficult to track down and does not provide the user with enough detail as to what field was causing the issue. We'd either like to see some validation on any fields that could cause or provide some data cleansing to strip this out before saving to the database.