Which email server is the most popular, recommended for nextcloud-snap administrators

Which local email server has been found to be popular, easily setup and able to be used with the nextcloud-snap installed application?

The Rain Loop email client looks like a useful email subscribers email client app as a add-on but requires a wild-card *@domain.name ability for 100+ users which could result in shelling funds out to a full business account on a hosted email server just for that ability alone. The alternative is a local email server that is fully capable of supporting a wild-card *@domain.name with 100+ user names.

Hence my query, which one would you recommend or have used that would specifically work with the Nextcloud-Snap installed application that can be very particular about internal settings and tweaks on its configuration to accommodate a external outside its sandbox application that is needed to run concurrently with it.