Mass Mail: DSN: undisclosed-recipients:;... List:; syntax illegal for recipient addresses

When I attempt to send a mass email to all registered users, I an error in my phpbb error log:

E-mail error
» EMAIL/PHP/mail()
/forum/adm/index.php

As well as this in my mail log:
May 1 22:45:22 server3 sendmail[27680]: o425jIUw027680: from=apache, size=3072, class=0, nrcpts=34, msgid=<a0dc600f6b05be56326eea3690fc4bc6@www.mydomain.com>, relay=apache@localhost
May 1 22:45:22 server3 sendmail[27680]: o425jIUw027680: o425jIUx027680: DSN: undisclosed-recipients:;... List:; syntax illegal for recipient addresses

I have seen some old tickets regarding similar issues but they are all marked as either "Wont Fix", or "Fixed", or "Unresolved".

This is an important issue for me.

Thanks