Cloudron Forum

You might want to take a look at the Messenger Matrix, which was compiled by security expert Mike Kuketz:

https://www.messenger-matrix.de/messenger-matrix-en.html

They are inexpensive but also professional. Changes to the name servers are implemented in seconds. I have had negative experiences with Netcup, for example, where changes took hours. You can also create multiple domain contacts and assign domains to your customers.