getting PHP error when sending mail, independent of transport method

It looks like your problem is not in this module. Somehow your symfony components are incompatible. LegacyEventDispatcherProxy isn't installed on my website, not sure where you get that from.

Don't think so, sorry.
LegacyEventDispatcherProxy.php is part of the Symfony coming with the Drupal installation.

There are a lot of other cases where modules have shown some incompatibility with the same error message:
https://www.drupal.org/search/site/Symfony%5C%5CComponent%5C%5CEventDisp... →

The site works flawless without this module, never had any issues. After installation, I get this error when sending when test mail. Nothing else was activated.

Could it be that the required psr/event-dispatcher version 1.0.0 is not fully compatible with the psr in Drupal 10.1.6 or that it is used in a wrong way?

Help is very much appreciated here, as this is way beyond my knowledge, sorry.

Anyone having any input here?

So I solved the problem.

In Drupal's /vendor-folder I had some conflicting symfony remains which weren't removed in a previous update.