What is the mechanism Exchange uses to transfer emails between servers in a co-existence environment? I'm going to be installing Exchange 2013 in one of my environments. Currently I'm running on Exchange 2010 (fully updated). I've got mail delivery directed to Exchange 2010 in the ASA. The goal will be a complete migration to 2013 but I want to keep the co-existence up and running for a bit to learn. How will external mail that can only get delivered to my 2010 server make it to a mailbox on my 2013 server? If anyone has any links to reference material that would be great.
TIA!