@antsu good, that's what I wanted to know. If it was like "off the shelf" services like FastMail, that generates DNS records so you can point your domain and everything is set up.
@cadusilva I haven't tried it yet. I'll test the waters for a few days with my "less important" domain before migrating my main one. In theory it should be very straightforward, just add the extra domain in the settings and create/modify the necessary DNS records.
@cadusilva I couldn't wait and ended up migrating over the other domain today too. One catch I didn't anticipate is that whatever you use as the MX record needs a matching PTR record, and you can only have one PTR, so all your domains will have the same MX (which doesn't even need to be in those domains, it can be associated with a 3rd unrelated domain). The only downside of this is that this MX host will be used everywhere for auto configuration, which is a bit annoying.
@cadusilva One other thing to be aware of: if you're trying to keep your domains somewhat "secret" from each other, Mailcow adds all the names in the same certificate and uses this certificate for all domains.
@cadusilva Trying to simplify a bit, imagine your server running Mailcow has the public IP 1.2.3.4, the fqdn mx.example.com, and a matching PTR.
You setup Mailcow with this as the hostname, then add the domains example.org and example.net. Both domains will need MX records pointing to mx.example.com.
When adding an account from one of these two domains to Thunderbird, it'll auto-populate the server field with that MX address instead of your domain.