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.