I am upgrading exchange 2010 to exchange 2013 and currently in coexistence. I cutover the mx to deliver mail to 2013 cas VIP last night and no mail was being received by the 2010 mailboxes, it was all being queued on the 2013 mailbox servers (mailbox and cas servers are separate in this deployment.) There are only a few 2013 mailboxes as I have not yet started migrating them. These 2013 internal mailboxes can not send to exchange 2010 either, but 2010 can send to 2013. 2013 to 2013 mail works fine. I have read somewhere about an issue with a receive connector having the same ip range that includes the mailbox servers, but I'm not clear on really which mailbox servers it is alluding to.
All the exchange 2010 servers are all on the 10.1.1.* network. Exchange 2013 CAS servers are all on the 10.1.1.* network and the 3 exchange mailbox servers are on 10.1.1.*, 10.1.2.* and 10.30.5.*. Only 1 AD site. On one of the 2010 receive connectors there is a scope of 10.1.1.0/24 I'm wondering if this is the culprit and by removing it should fix the problem. Any other ideas would be appreciated.