[Dovecot] Connection closed by foreign host.

I'm writing this on behalf of someone else...

They successfully setup Dovecot, and can login to Dovecot using telnet but as soon as they are logged in, they get the message "Connection closed by foreign host" and are disconnected. The user is a valid Linux user and the mail file exists under /var/mail. They say no errors are written to /var/log/maillog.

# telnet localhost 110 Trying 127.0.0.1... Connected to localhost.localdomain (127.0.0.1). Escape character is '^]'. +OK dovecot ready. USER mymailuser +OK PASS 123456 +OK Logged in. Connection closed by foreign host. #

Can anyone give me ideas where we would begin troubleshooting this?

Many thanks, Steve :)