Outbox not sending emails

Outbox not sending emails

Two of us at same company mysteriously wondered why responses weren't coming from emails.  Discovered the Outbox hadn't sent email for about 3 weeks (both of us) starting at about the same day. 
Message: Authentication Failure: Kindly change the Mail settings and re-enable auto fetch. 
    Finally found some setting was set to disable, and reset it to 'enable' but old emails didn't send; seems to be working OK now.  Any thoughts out there?