I am working on a SBS 2008 server with Exchange 2007, and mail for the most part is working well. This particular office has the ISP host their mail, and the exchange server pulls the mail down to their local exchange server. I discovered they prefer this arrangement because it allows them to continue sending and receiving mail should their mail server go down for any apparent reason. That makes sense and sounds like a good idea.
The problem is certain outbound mail is not sending out for specific domains. The messages sit in the mail queue indicating "Mail Delayed", but then a couple days later drops the mail message stating delivery is delayed and has not yet been delivered. Then a couple of days later they get another email indicating the local exchange server has been trying to deliver the message without success and has stopped trying.
I've am at a loss in trying to figure out how to fix this. I've ensured the Internet router is not blocking anything outbound, disabled anti-virus software on the exchange server, ensured the ISP has the correct Reverse PTR records, conversed with the party we’re trying to send mail to ensure we're not blocked on their end, etc. I even created a new Internet Send Connector. I spoke with the ISP about this problem, and I had them allow me to setup a smarthost temporarily and when I did the mail sitting in the queue immediately sent out. This ISP does not allow smarthosts for client exchange servers when they host their mail. They only allow it for clients that have their mail hosted by another ISP. So I was forced to disable the smarthost after the test. The only thing I can think of is certain Spam programs might be checking the IP, hostname, and MX record. Maybe it’s failing with the MX record because it shows the ISP instead of the local exchange server. I am at a loss here. The ISP stated we have two options: use the ISP popmail exclusively or have the exchange host the mail. They also mentioned using a smarthost is not an option and the connection will be shutdown if you choose to use one. Is there anyway around this by allowing them to keep this mail configuration of the ISP hosting the email without using a smarthost? Maybe we could create a unique send connector or something. Not sure, that's why I am posting this question.