[mail] migrate to blahaj.land

About this issue

While Heliohost has served us well through their Tommy donor server, we sometimes experience issues regarding load and webserver errors on the webmail. Since I decided going to Blahaj.Land / Shork.eu for mailboxes, why not let's also bring our mailbox there too.

Currently, custom domain mailboxes require manual setup on the mailcow side of things but once ready we can configure them in the admin dashboard.