by default, Exim puts tabs in the "Received:" header to fold it.
Recently, I learned that folding is a transport encoding, and clients
should unfold lines when displaying messages. Contrary to common belief
and various implementations, unfolding means only to remove the CRLF -
no more (RFC 2822). In particular, compressing white space at the fold
mark is not allowed.
An unfolded "Received:" header with tabs looks quite ugly. For that
reason, I suggest to fold the header by a single space instead of a tab.
I know that various clients either display the CRLF, or compress all white
space, no matter if spaces or tabs, to a single space. That behaviour
is broken and should be fixed. The IETF Sieve WG discussed this issue
and came to the conclusion that Sieve should follow RFC 2822. So at
leasts some people don't want to engrave those bugs in rock. :)
While talking about changing the default, I suggest to add:
