I am in the middle of a migration from 2010 to 2013. Actually, this started from 2003, but the 2003 to 2010 went very smooth. The problem is not all the time, but intermittent. I would say it happens more often than not though. If I send an email from a 2013 account to t 2010 account, the message is delayed for up to 15 - 20 minutes. The same is true of email from 2010 to 2013. Initially mail between 2013 accounts seemed to be fast, but now that is delayed as well. I did some research, but I have not been able to come up with any reason for this. Sometimes the email goes through immediately, but more often it take 15 - 20 minutes. This can also happen with email from the internet to a 2013 account. I am running Exchange 2013 with CU7.
Any help will be appreciated.
Phil Goldwasser