Jeff, what version of Exchange are you using? We are on 2013 and I was getting the similar errors on the POP3 side as well as problems sending on port 25. To fix the POP3 I had to reset the password to only 8 characters and it started working fine (When the password was longer than 8 chars. one of the errors I was getting was 'password cannot be null' so maybe there is a problem with how the password is saved). I also switched to incoming server type Microsoft EWS which seems to work much better with Exchange 2013. For outgoing, I selected 'Use different credentials for SMTP' under Advanced Options, then left the username and password blank.
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