[Exim] Upgrading from Exim 3 to Exim 4

Hello,

I just upgraded our fairly large mail system to Exim 4. The migration
was easier than I expected. The conversion tool did a good job to give
me a starting point for the new configuration and fixing the remaining
issues took me perhaps an hour per configuration file. One thing that
isn't obvious is the new way require_files is parsed. Perhaps the
conversion tool could at least put a warning before that option.

All in all, upgrading to Exim 4 went pretty smooth and it works great.

Michael