All outside calls go to the Unity_servername mailbox, which should reside on the partner mail store server you configured in Message Store Configuration Wizard. From the Unity server, you can use Exchange System Manager to view the mail-enabled accounts. You should check to see if the mailbox is there. You may want to run SYSCHECK logged onto the Unity server as your unitymsgstoresvc account and see if it reports any errors. Did you by any chance move the mailbox to another server? One last thing I would check - in Active Directory Users and Computers, with Advanced Features selected in the MMC so you can see the Security Tab, check to make sure the unitymsgstoresvc account has the required permissions (and that inheritance is enabled/checked on the Advanced options of the Security Tab) and lastly, that the unity_servername account has not been disabled.
These are the paths to get to each CCX logs through CLI. They may be helpful if you are having issues accessing RTMT or downloading logs through it.
If you want to download them you have to prefix "file get " and you can add one of the options (re...