In our setup users require a specific LDAP group to login. We enforce this
with an LDAP filter (user_filter). With DN template, authentication always
succeeds for these users, as Dovecot only performs an LDAP BIND request.
Successful auth followed by failed userdb lookup seems to trigger a bug in
Dovecot that causes the handler processes to stay around indefinitely and fill
up the process_limit. Using DN lookup with the LDAP filter set for both
user_filter and pass_filter should cause the authentication to fail for these
users and work around the bug.
