Why doesn't logwatch parse Dovecot 2.x log messages properly on Red Hat Enterprise Linux 6.2?

Issue

• Why doesn't logwatch parse Dovecot 2.x log messages properly on Red Hat Enterprise Linux 6.2 ?
  Following lines are found in the daily logwatch mail.

     **Unmatched Entries**
        dovecot: auth: pam(usera_01,x.x.172.70): unknown user: 245 Time(s)
        dovecot: auth: pam(userb_01,x.x.105.118): unknown user: 1 Time(s)
        dovecot: auth: pam(userc_04,x.x.25.251): unknown user: 2 Time(s)
        dovecot: auth: pam(userc_23,x.x.25.251): unknown user: 2 Time(s)
        dovecot: auth: pam(userd_18,x.x.42.218): unknown user: 1077 Time(s)
        dovecot: auth: pam(usere_57,x.x.228.126): unknown user: 244 Time(s)
        dovecot: imap(userd_36): Disconnected: Logged out bytes=270/46224: 2
    Time(s)
        dovecot: imap(userd_36): Disconnected: Logged out bytes=29/405: 1 Time(s)
        dovecot: imap(userd_36): Disconnected: Logged out bytes=70/469: 1 Time(s)
        dovecot: imap(userd_36): Disconnected: Logged out bytes=82/437: 1 Time(s)
        dovecot: imap(userd_38): Disconnected: Logged out bytes=12/342: 1 Time(s)