I am migrating an email server from SBS2003 to Exchange 2010. (I know it's old.) In testing co-existence with the old server, I have migrated 5 users from the old 2003 to the new 2010 server. These two servers are behind a firewall, with one external IP address. Migrated users are receiving email. Mailboxes on the old server are also sending and receiving fine.
However, when someone from comcast.net sends an email to firstname.lastname@example.org on the old server and email@example.com on the new server, comcast.net never sends the email. I have tested this on their portal, and from Outlook. I have tested email from a gmail account which works fine.
I am getting temporary and permanent connection errors from comcast.net.
While I am trying to work with Comcast, is there anything I can do to determine what the cause of this issue is?