Steps to be taken if Exchange 2010 does not see the besadmin account as MAPI after Exchange 2007 to Exchange 2010 migration.

Open EWS(exchange powershell) on Exchange 2010 server and enter the following command:

Set-RpcClientAccess -Server EXCHANGESERVER -EncryptionRequired $False

Exchange 2003 and 2007 allow rpc requests by default, but 2010 directly denies them. For this reason, with the above command, we allow RPC, that is, “remote procedure call” requests. Since ExchangeMAPI, which is a prerequisite in the bes installation, does not have rpc support by default, the mapi client goes to 2010 exchange and cannot get permission. As such, it reads the besadmin account from the server it sees as the last positive.

Hope it will be useful.

Thanks to Alper Yazgan.