A few days ago, it was brought to my attention that a user was experiencing delayed NDR’s from our email server when sending emails to Yahoo. It has happened in the past where this same problem seemed to have been intermittent and system wide. After some time had passed and I was not able to come up with a solid resolution, the emails would suddenly start reaching their destination. I exhausted every single source pertaining to a 421 4.4.2 error which all had failed and even questioned our ISP who handles our DNS entries.
Most recently, I ran into this issue again and decided to see if our Watchguard XTM 510 box was the culprit which come to find out, it was!
I was using a proxy for outgoing emails for logging and other features that you do not get with a packet filter. When I switched our email policy from a proxy to a packet filter, all of the emails that were stuck in queue instantly cleared up.
Since I was able to pinpoint the source of my problem, I want to know why. Even after disabling everything and allowing all in the proxy, I could not send an email to Yahoo using a proxy.
Do I even need a proxy policy for outgoing messages? If so, what am I missing that is preventing emails from being sent to Yahoo’s domain? Would using a packet filter be any less secure for outgoing emails?
Any help would be greatly appreciated.
Exchange 2010 (CAS, Mailbox, Hub)
Watchguard XTM 510 (v11.6)