The greylistd package currently recommends exim4. I wrote a filter for
OpenSMTPD that uses greylistd for providing greylisting services
(shameless plug: https://github.com/jdelic/opensmtpd-filter-greylistd/).
With the current package, if the user tries to install greylistd without
--no-install-recommends, apt will happily uninstall OpenSMTPD to install 
exim4. This is not ideal from a user-experience point of view.

I think the best solution is for greylistd to merely _suggest_ exim4. If
we want to keep the recommendation, an alternative solution would be to
recommend the "mail-transport-agent" meta package. This way OpenSMTPD
would also fulfill the recommendation.

What do y'all think?