I previously used email ext plugin version 2.71 to send mails. Since upgrading to 2.73, the server is not able to send mails anymore. Also 2.74 did not fix it.
In the log it looks like this (for 2.71, 2.73 and 2.74); Mail anonymized:
07:30:51 [Pipeline] emailext*07:30:52* Sending email to: firstname.lastname@example.org*07:30:52* [Pipeline] }
But only 2.71 actually is able to send the mail. No settings/configurations/pipelines were changed and I switched between 2.71, 2.73, 2.74 multiple times to ensure that it is not a temporary problem.
Workaround: Downgrade to 2.71 for now.