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
https://www.rfc-editor.org/rfc/rfc2822
其实无卵用。