We are trying to setup exchange 2010 along with our current email server which is Imail Server 12 and its causing problems with users not getting emails at different times. I only have 4 exchange mailboxes created just to try it out before putting it in production for everyone to start using when we switch over from Imail to Exchange 2010. Most of the time here lately when i have exchange running it coming back with an return error of 550 5.7.1 error code. I can send emails to public addresses but when you reply to the messages thats when i get this error. I check with ipswitch and downloaded a article they had on it and configured the imail for exchange which you have peering imail with exchange but the problem is in the configuration of exchange. Maybe with the accepted domains or receive connectors or authentication process.