Thanks so much for this mod; it's really helpful for our forum!
One bug we encountered: when more than one person is @mentioned in a post, I think the first email is correct, but subsequent emails have a body like:
"Joe mentioned you in the post "Bob, you have been mentioned at a post in Your Forum", you can view the post at http://..."
instead of giving the subject of the thread.
I fixed this by editing Sources/Mentions.php - function mentions_process_store
The problem is that $subject gets overwritten on each loop.
After line 237 I added a new line:
$topicsubject = $subject;
Then I changed the current line 253 from
'POSTNAME' => $subject,
to
'POSTNAME' => $topicsubject,
I think that fixes it!