Your software
mautic 3.1.0 (from github)
php 7.3.22
Your problem
When an email address is wrong (does not exist) and you send an email manually via the contact, the error message is showing up on top. Since the send button is all the way down, You can not see the error message and will wander what’s goin on.
Failed to send to c----@domain.tld: Expected response code 354 but got code “554”, with message "554 5.5.1 Error: no valid recipients "
Screenshot with the send button. the error message is shown above the green box and not visible when you hit the send button. So the feedback (error message warning) is at the wrong place.
Screenshot: If you scroll back up you will finaly see, why nothing is happening after hitting the send button.
These errors are showing in the log: