Question

Why is the php documentation of mail() saying

Lines should not be larger than 70 characters.

(under the part: parameter message)?

Is there an RFC specifying the line length of mails?

This question:
PHP mail and Lines should not be larger than 70 characters says

there are not that many clients left that cannot deal with long lines

So is it just bad build clients/servers that do bad stuff to too long lines,
and 70 is a tested value that works well enough, or is there an RFC or something that
says that mail-applications need to handle 70 char long lines?

Answer 1

The Internet Message Format RFC the latest of which is 5322

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 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

(The MIME RFC also specifies a max of 76 characters)