@Stefano: So far I hadn’t used SMTP. I tried to activate the SMTP function yesterday with two different sets, but both produced the same error. But I’m going to give it another try tonight, maybe I did the settings wrong.
If I contact the provider, what shall I tell/ask him?
I really wonder what happened, I wasn’t informed by my provider of any changes since September. (in September the newsletter mailing still worked).
Thank you for the support! Highly appreciated!