POP3 server doesn't support "reauthentication"

The USER and PASS commands end up logged as unsupported commands and rejected by the server:

  4/15  12:03:46a  9476 POP3  <User Name> !UNSUPPORTED COMMAND: 'USER user name'
  4/15  12:03:46a  9476 POP3  <User Name> !UNSUPPORTED COMMAND: 'PASS secret'

Once authenticated, these commands are rejected by the mail server and that can confuse ThunderBird as it sometimes reuses the same already-connected TCP socket to get new mail and attempt to authenticate a second time.