I don’t see any e-forum and don’t even see an email there, so I visited their commercial e-site Servercow.de and there was an e-mail address there.
Mailcow also like Miab uses Roundcube https://mailcow.github.io/mailcow-dockerized-docs/third_party-roundcube/ and this Roundcube is a heavy weight software to tweek, this guy did it in 2006 and it was not easy https://www.roundcubeforum.net/index.php?topic=135.0
https://modoboa.org support plans
Ouch! Price €800.00 €1,200.00 €3,200.00 €7,500.00