- Some MTA do not send messages when the domain part of the E-Mail address is different from the domain the site is hosted on.
This is not the case with us, messages from Joomla and Kunena are working fine.
- Some servers do not send messages when a full email address is used, try the username without "@domain".
See above
- Try mosmail instead of phpmail (or vice versa).
switching from phpmail to mosmail seems to have done the trick - thanks!