I am in the middle of a migration from Exchange 2010 to Exchange 2013. Everything seems to be correct and I can send email from and to users on the Exchange 2013. Mail from and to internet addresses with an exchange 2013 box is fine. Mail between 2013 exchange accounts is fine. Mail from an exchange 2013 account to a 2010 account is fine. Only mail from a 2010 account to a 2013 account is slow. I cannot figure out what is causing this slowness. The messages seem to take around 10 minutes to arrive in the 2013 box.
Any help would be appreciated.