our company E-mail is handled internally by Exchange 2003 on a Server 2003 SP2 box, up to date with Microsoft. Our firewall is a SonicWall NSA-3500 which routes incoming port 25 traffic to a hardware-based SonicWall ES-300 mailfilter. Firware on both of the Sonicwall units have been updated.
For a month or so, E-mail has been sporatically delayed from certain senders. The domains from which they are sending are quite different. It started with 1 user noticing 1 delayed E-mail and seems to have gotten progressively worse.
I've attached a packet capture (screenshot) taken from our firewall. The screenshot shows (starting on line 2) the source IP X.X.45.253 SYN is received by our internal mail filter X.X.1.60 and our mailfilter sending the ACK. Line 4 shows a TCP previous segment lost which translates to "the sequence number received is greater than expected" according to Wiresharks website. This sender's E-mail is always delayed at least 2 days if not bounced back to him entirely. I'm working with a technician on the E-mail sender's side as well as with SonicWall. The technician can Telnet to our mailserver from his IP X.X.45.250 but cannot Telnet to our mailserver from his IP X.X.45.253 which is the IP address of his mailserver.
We really need to get this resolved ASAP. If you have suggestions please let me know.