rfc2822 April 2001

2.1.1. Line Length Limits

   There are two limits that this standard places on the number of
   characters in a line. Each line of characters MUST be no more than
   998 characters, and SHOULD be no more than 78 characters, excluding
   the CRLF.

   The 998 character limit is due to limitations in many implementations
   which send, receive, or store Internet Message Format messages that
   simply cannot handle more than 998 characters on a line. Receiving
   implementations would do well to handle an arbitrarily large number
   of characters in a line for robustness sake. However, there are so
   many implementations which (in compliance with the transport
   requirements of [RFC2821]) do not accept messages containing more
   than 1000 character including the CR and LF per line, it is important
   for implementations not to create such messages.

   The more conservative 78 character recommendation is to accommodate
   the many implementations of user interfaces that display these
   messages which may truncate, or disastrously wrap, the display of
   more than 78 characters per line, in spite of the fact that such
   implementations are non-conformant to the intent of this
   specification (and that of [RFC2821] if they actually cause
   information to be lost). Again, even though this limitation is put on
   messages, it is encumbant upon implementations which display messages
   to handle an arbitrarily large number of characters in a line
   (certainly at least up to the 998 character limit) for the sake of
   robustness.