Please check the following:
1. Make sure the server account is not hidden from the GAL or is disabled,
2. Make sure the service account does not belong to any groups such as Domain Admins or Exchange Admins which have explicit "receive as" and "send as" denies. The service account does not need to be in any groups.
3. Re-enter the password for the service account into the source server properties. Select the Source Exchange 200x Organization and browse down thru it until you get to the source server. Right click on the server and select Properties. On the screen you will see the Exchange service account. Select the Modify... button and re-enter the service account password. Select the OK button. You may get a pop-up massage saying that the setting needs to be re-applied. Go to the mailbox job and commit the changes and make sure the source agent is running.
4. Make sure you have given the service account Full Mailbox Access as per page 36 of the System Requirements and Access Rights guide (version 8.5).