Huge delay in mail delivery!!

==== Required information ====
- iRedMail version: Pro 1.6.0
- Store mail accounts in which backend (MySQL):
- Linux/BSD distribution name and version: Ubuntu 12.04.2 LTS

2 Domains
1600 Mailboxes
====

I have just migrated from a Sendmail server to a new iRedmail server setup.  Mail is flowing, however, I am seeing about a 30 min delay on all message delivery.  I've played with the greylisting a bit to deal with several Black Listed friendly IP/Hosts.  Any idea what I might have going on?

Thank you,

tonyd