Multiple E-Mails for Thread Subscriptions and PM's
 

We're running vB 3.8.4 on a system with 3 apache servers and 1 data server. Currently for e-mail, we're using the php mail option in vBulletin.

I've had several members complaining that they are receiving several e-mail notifications for thread subscriptions with "instant notification" sometimes they are getting notified from all 3 servers at the same time.

Any thoughts on how to remedy? BTW - using the pop mail isn't a viable solution, because for some reason, our pop mail is blacklisted by a major internet provider, but the php mail gets through.

Thanks