We recently installed Exchange 2013 Enterprise Server to upgrade Exchange 2010 Server. After installing and configuring new Exchange I migrate a few users as a test (internal DNS and NAT still points to the old server). I was able successfully send e-mails back and forth between servers without much of a delay. Then I changed internal DNS as well as firewall settings to point the Exchange 2013 server. Users on Exchange 2013 server are still able to receive e-mail without delays but when they reply, it takes about 20 minutes for the messages to be received by the users still on Exchange 2010. Same problem from external users as well.
I checked all the setting and I can't find any issues. Does anyone have any ideas what might be going on?
Thank you,
Serge