Problem: Mail is being randomly delayed (getting delayed notifications) to some people and it appears to only be when the server has had its traffic routed out the FiOS line; mail routed out the Cable line is fine. No failure messages to date (I suspect that eventually, the message is routed out of the cable line and then accepted by the recipient).
(The IPs below are NOT the actual IPs - I have changed them to start as a private IP but actual IP is a recognized public IP)
Cable (original ISP) with static IP of 10.254.166.124
FiOS (new ISP for redundancy)
Attempting to load balance between them.
Public DNS has A records and MX records as such:
mail.mydomain.com A 10.254.166.124
mailfios.mydomain.com A 10.1.147.34
Public DNS has an SPF Record of:
mydomain.com IN TXT "v=spf1 mx ip4:10.254.166.124 ip4:10.1.147.34 -all"
If you telnet to the server's SMTP port on either IP you get
220 mail.mydomain.com Microsoft ESMTP MAIL Service ready at Sat, 26 Feb 2011 15:15:58 -0500
(To be clear and why this is significant, it does NOT answer mailfios.mydomain.com, if it did, I presume we would then be having problems with mail sent out the cable line)
Router is a Fortigate system with the latest 4.0 MR3 firmware and configured to "load balance" by sending up to x bytes out the cable line then, sending y bytes out the FiOS line, so at any given time, the server COULD be sending e-mail from EITHER the FiOS IP or Cable IP.
NOT ACCEPTABLE Solution:
Any suggestion to alter the load-balancing config so that the server ALWAYS uses one ISP unless that ISP goes down. While this would work MOST of the time, it would start creating problems if and when the line does go down. Even temporary problems are unacceptable.
The ULTIMATE QUESTION:
How can we get this to work (change SPF, change SMTP banner, change load balancing (doubtful)) so that the intended recipient mail servers DO NOT delay or reject our messages? Why are they being delayed?