问题
RFC 2045 defines the maxmimum line length for encoded data as 76 - however I cannot find any explanation as to why it is 76. Is this number entirely arbitrary, or is there some reasoning behind it?
回答1:
RFC2822 is legacy standard of EMail. In section 2.1.1 of RFC2822, you can find reason as below: It also affects MIME.
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.
回答2:
Actually the original RFC 822 defines a limit at 72 characters and the culprit is a teletype, which was a standard output device with the early computers.
You can also "thank" teletype devices for the line terminator in emails (and Windows) being 2 characters, which are CR (Carriage Return) and LF (Line Feed).
It was essential to transmit this sequence at the end of each line in order for a teletype to move a caret to position 0 and advance paper one tick up.
By the time RFC 2822 obsoleted the original, nobody was using teletypes to render emails, so it was relaxed a bit in order to fit into a default TTY monitor device.
回答3:
The bit to do with user interfaces
http://en.wikipedia.org/wiki/Text_mode#PC_common_text_modes
Basically, 80 characters across (and usually 25 or 30 lines) was the most common standard for displays. 78 provides a sane standard as this allows for some small decorations to be used (borders).
来源：https://stackoverflow.com/questions/4994447/reasoning-behing-76-being-the-line-length-limit-for-mime-sections-as-defined-by