hugogr45 That might also explain why port 25 is unavailable. Perhaps they do not want mail servers to be operated on these hosts!? Or, maybe you need to contact them to set the PTR record and open port 25.
In any case, to operate a mail server, you need a corresponding PTR record, and port 25 must be open for both outgoing and incoming traffic.
Double check in the Ionos admin panel to see if a firewall is active. If it is, make sure that port 25/tcp is allowed. Also check again whether you can add a PTR/Reverse DNS record anywhere in the admin panel. If not, contact Ionos and ask them to set it for you.
Should it indeed not be possible to set a PTR record and/or open port 25, then the provider or the particular plan you are using is simply not suitable for hosting a mail server.
