[exim] Disk failure => 5xx error code

Before anything else, thanks to the exim authors for all their hard
work.

We had a problem yesterday when a disk failed and exim started rejecting
all mail with a "Do not retry" (5xx) error code. The text indicated a
"no such user" error. This stopped our upstream relay from trying our
alternate host.

Whilst appreciating the problems with getpwnam error codes, the fact is
that some OSs do return error codes for IO errors, etc. Could we not at
least check errno for any of the known error codes indicating hardware
or OS problems after a getpwnam failure? Such a function would be
trivial to write although integrating it into the code would require a
little more knowledge of the exim codebase than I have.

I really want a total disk failure to cause mail to go to my alternate
host not just bounce!

Thanks again.

John