It appears that the PHP mail command in combination with the postfix MTA will automatically wrap lines that are longer than about 998 characters, as far as I could ascertain. I believe ZF in all version uses mail when using the Sendmail transport.

I am not sure about other MTAs or other transport methods (e.g. SMTP).

While you have more control when wrapping the lines yourself, in certain configurations you do not NEED to wrap lines. It is best to just test this yourself. As others have pointed out, the limit of about 80 characters is just a recommendation.