I don't pretend to know how every mail user agent (client) handles "line end" characters but as I recall the original RFC821 specification allowed up to 1000 bytes per "line" in the DATA portion of the SMTP transaction. I haven't seen it lately but a few years ago I know some of the mail programs being used here used to end up with huge lines (like 32KBytes) of blanks or nulls or something like that. But I guess it could be a case of the lines being pasted not being terminated too... I hope others have more details.