So this has bitten me for the past couple of weeks we would try to fail over our last two mailboxes from exchange 2010 to exchange 2016. One Journal and One Service account. Come to find out (No thanks to EMC SourceOne install documentation) That the service account needs to be the last mailbox to move.
Take a look at this bug report: https://community.emc.com/docs/DOC-60862
If you are going to run in a mixed environment for a long time follow the steps below in the article and always make sure you migrate your SourceOne Service account last!
With 2010/2016 co-existence, there are certain requirements for it to work.
These requirements are:-
1) Service Account mailbox needs to be always on the lower exchange version.It should be the last mailbox to move to 2016. Exchange 2010 should be configured with RPC over https.
2) Exchange 2016 should have RPC over https enabled. By default Exchange 2016 has MAPI over https which is not supported by SourceOne. Customer will have to enable RPC over https and configure it correctly
3) All ES1 servers should have only Outlook 2010 SP2 + KB2878254. No other HF or update. Updates for Outlook should be disabled.
4) Profile needs to be configured on Outlook to have Outlook using RPC over https with the proxy server pointing to the 2016 server. For these changes you will need to configure AutoDiscovery.
5) Set the correct permissions mailbox on 2016
Without these 5 criteria, they will not be able to communicate between 2 versions of exchange.