We have exchange 2003 (Version 6.5 service pack 2), when we try to send email to one of our client it always remain in Exchange queue , we get Delays email at specific interval, but no NDR and ultimately it never reaches to that client. We have email filter system for our exchange server but only for incoming email and not for outgoing email.
At the client side, they can send us email without any Problem from their side. They have got Exchange 2007 behind Watch Guard firewall, firewall has got Anti SPAM enable as SMTP proxy for incoming email. But when we checked their Watch Guard box it does have our domain blocked anywhere, and it is not suppose to be culprit as that box always send message back to sender that the email has blocked by XYZs watchGuard SPAM filter.
Moreover, the user at client side has configured her Gmail account to her local Outlook as POP3 account and that is also behind the same firewall, but when we send email from our domain to her Gmail account she can receive it to her Outlook through same firewall.
I do not know what could be wrong and where else to check as we are not facing this problem with any other recipient but this particular domain. And they also not having problem with any other sender but only from our domain.
Any help would be greatly appreciated.