This happened at lang.moodle.org: Helen did not receive any email copies of forum posts. After a long and non-trivial debugging and tracking the issue (requires access to maillog which is almost impossible for mortal admins on shared hosts) I realized that the problem was caused by connection timeout with Jabber server (will be reported as separate issue).

It seems to me that if a message processor (eg jabber in this case) fails it stops the whole message processing. So the message is not sent via email either - even if it could be.