Dovecott: Internal Login Failure

Problem

# telnet localhost 110
Trying 127.0.0.1…
Connected to localhost.localdomain (127.0.0.1).
Escape character is '^]'.
+OK dovecot ready.
user $USERNAME
+OK
pass $PASSWORD
+BYE internal login failure. error report written to server log.

Solution

In /etc/dovecot.conf , check the value of first_valid_uid -- by default it is 500, meaning that users with a UID of 499 or lower can not log in.