I am in the process of migrating from Exchange 2007 to Exchange 2013. I have installed my 2013 Cas/MBX into my environment with the existing 2007 Hub/MBX and existing 2007 Edge. Mail flow on the 2007 side is unchanged (working) but on the 2013 side I am able to receive mail but I can't send (from test mailbox). It just sits in the que on the 2013 CAS/MBX until it expires.
Any ideas? My Exchange 2007 servers are both upgraded to SP3 Rollup 13 so the versions should be fine.