vBulletin sending phantom emails?
 

I recently signed up with SendGrid to outsource outgoing email and to obtain outgoing email deliverability metrics.

One thing SendGrid provides is a bounce report. I recently reviewed the bounce list activity for the last month and it appears that vBulletin is sending out emails to users who have no posts, no subscriptions, and who have not visited the site in years.

I am basically dumbfounded because I have not sent out any bulk emails to users and I can't think of any type of emails such users would be sent by vBulletin.

Any ideas or things I am not thinking of?