Monitor email queue on SMTP server

I need some help in creating a custom monitor that will probe the SMTP servers and check for the email queue length. If its above threshold, take the server out of load balancing algorithm until the queue is cleared. This is to alleviate the issue of emails being queued in one Data Center for some unknown reason. The idea is divert the email traffic to another Data Center if emails are queuing up in one Data Center.

I believe this can be achieved via an external monitor using SNMP? or any other thoughts?