as you told me that I got something wrong, just help me to help you :)
Mail and DNS hosted with Network Solutions. MX records only point to NetSol.
Exchange POP connector used to download mail to our Exchange mailboxes.
Both is ok.
Exchange 2007 uses direct DNS for all outgoing mail. Mail that is sent from our public IP (220.127.116.11) >is delivered to the spam folders for providers such as Comcast and yahoo. When they reply to that >mail, it is never received by us.
This looks a lot like a wrong setup of the MX-records or an error somewhere in the mail system. Normally the e-mail should hit at your providers mailbox and then you should get it via pop3 connector. As this doesn't happen. Check the MX-records and check any spam folders available at your providers mail system. (POP3-connector isn't downloading subfolders iirc).
If we login to the NetSol webmail portal (mail.ourdomain.com), and send mail to Comcast or yahoo, >the message is received normally, and we receive replies. My thought is that once the message is >marked as spam, their reply never gets to us, as the NetSol spam filters are extremely aggressive, >and drop the message. We have no control/access to these filters or logs of what is filtered. But >why are we being marked as spam?
I'm pretty sure this problem comes from wrong mx- or spf-records. When you send an e-mail via the netsol web ui, it's delivered ok, because it's delivered via the netsol smtp which is setup correctly in dns...
Currently, our SPF looks like this: v=spf1 ip4:18.104.22.168 mx a:inbound.ourdomain.com.netsolmail.net ?all
MX: inbound.ourdomain.com.netsolmail.net.(10) with CNAMES of mail.ourdomain.com and smtp.ourdomain.com
The reverse DNS entry on our ISP is ourdomain.com for our IP
"Our IP"? From the webserver of from the mail system?
The banner HELO response from our server is <220 ourdomain.com>
Our IP address is NOT blacklisted anywhere, and the Cisco Senderbase gives us a score of Good
As I wrote before it has something todo with your dns setup. But I'm only able to check it if you give me the domain we're talking about.