I have exactrly the same problem.
What it is strange is that for years I could send a mail test under joomla/global configuration/server/e-mail configuration, but now It does not work either. More strange is that if I install Acymailing 6. Sending a test works again, but we do not want to use Acymailing 6 until it is more polished. In addition, now we have 2 acymailings installed!