I have a client running a server:
Server 2003, fully patched
Exchange Server 2003, fully patched
No other services or roles on this server.
Static IP, no RBL listings (verified via DNSStuff.com and MXToolbox.com), proper rDNS and PTR records (verified via DNSStuff.com).
There have been no changes to the server, ISP, IP, etc. and over the past week a lot of the users are receiving Exchange Delivery Status Notification, message has been delayed.