Sending email fails, though Test-Email works

Nextcloud Hub 3 (25.0.13) on Debian 11.8.

System is some 10 month old and sending emails has always worked. I guess that since the last update it is not possible to send email notofications (f. ex. welcomemail to new users).

With the settings in Admin => Grundeinstellungen it says “OK” and I receive the Test-Email.
When sending emails via the users => “Willkommens-Mail erneut senden” it says “sending email failed”. Logs are saying:

Swift_TransportException: Expected response code 354 but got code “554”, with message "554 5.5.1 Error: no valid recipients "

Any ideas? I tried different settings in the GUI as well as in the config.php, with no luck :\

Thanx in advance!


We coud solve this by using another mail server with another account. The problem is the PHP-Sendmailer, I think.

  1. When using the same account f. ex. in Thunderbird on another system, it works with the same settings. SSL-certificate is valid (let´s encrypt).
  2. Where could we log the activities of the sendmailer? Because there is nothing to see in the logs of the receiving (SMTP-)server, at least not with default loglevel.

I assume that the PHP-sendmailer in NC has problems with some StartTLS-implementations. I t works now, but it would be nice to find the reason for the error.