It appears that the notifications for openacs.org will not work properly due to an error in DNS for openforce.net (which hosts the openacs.org system).
The Postfix mail server error is:
Apparently the server advertises itself as being 126.96.36.199.openforce.net ; yet, if you try to ping this hostname you get a host unknown error.
From what I can tell, simply changing the DNS to add an A record for 188.8.131.52.openforce.net pointing to -> 184.108.40.206, will work. It seems that the reverse DNS is setup properly (PTR record), but not the A record.