I'm trying to make sense of rfc5322 Line Length Limits . Is the line limit 78 chars or 998 chars? Is one for the body and the other for the headers? I can't find anything to specify that.
Each line of characters MUST be no more than 998 characters, and SHOULD be no more than 78 characters, excluding the CRLF.
It's saying that preferably, lines should ideally be no longer than 78 characters but lines must never be longer than 998 characters.
In other words, 998 is a hard limit while 78 is a soft limit.