mail.audioholics.com resolves, and isn't on the blacklists..
Server side can check:
-outbound email isn't get internally limited
Inside vBulletin
-under the admin panel go to setting > options > email options and see if mailqueue cron based sending is active.
Something i've seen is vbulletin's get migrated (like to a new server, or a host does an update, etc) and the cronjob for sending mail notifications gets lost (because its server side and therefore won't be included when copied over). The regular email continues to work, even vBulletin's internal test works, but the mail somehow gets lost with the missing cronjob the software set on the previous server. The fix is as easy as resetting.