I have to tell you the same.
My older version which worked without a problem before always showed me, that the mail would be sent.
After many months of vb updates and other new addons a user asked why there was no commbull for such a long time.
After testing upgrading, downgrading and upgrading again, some user received mails some not.
And after testing of the versions i've got "mail delivery failure" email sent from a commbull sent 5 days ago.
I thougt an addon is causing the problem but you can never know...

Just wanted to mention that.
Normal forum emails are sent as ususal (subscriptions, activation mails and so on...)
I'm still checking!