After a mostly sucessful migration to Exchange 2010 over the weekend, today was the day to deal with the fallout. The big issue we found was that SharePoint was not able to send emails to external (non AD) email addresses.
I had an "Internal Relay" Recieve Connector setup with all my internal IP addresses of servers and other devices (scanners, etc) that need to use the SMTP server. Our SharePoint server was on the list but still I was unable to get SharePoint to send email externally. To test if the server could connect to the Exchange server I used telnet (see here for how to test: http://blogs.technet.com/blairb/archive/2008/05/08/unable-to-send-e-mail-alerts-to-users-with-an-external-email-address.aspx)
If you get an error like "550 5.7.1 Unable to relay for emailaddress@domain.com" then something is wrong with the connector. In my case, this wasn't related to the Authentication or Permission Groups tabs, it was related to the Network. By default the connector had (All available IPv4) for the local IP addresses to recieve mail. You would think this would work but not for me. Once I changed this by adding in the IP address of my Exchange server manually, emails were able to be sent externally.
I scoured the internet and couldn't find anything that had this explanation so here is my blog to help others.
