thank you for your reply!
Well, I tried it with the diagnostic panel this morning where it said OK. Now it says:
Email sent with WordPress: FAILED (Could not instantiate mail function.)
Email sent with Newsletter: FAILED (Could not instantiate mail function.)
I also tried with a different e-mail address in the general settings (not the same domain name within the address) but it did not work at all.
Here is some content from the return to sender error:
Temporary MTA failure on relaying, From MTA() during
fwd-connect (All attempts (1) failed connecting to smtp:*:10025)
I have no idea if the message tells you something…