Your cron and cdb settings are verymuch valied but technically it is not a good Idea. clearopensmtp will not clear the relay opened for that IP. You need to check the cdb database either it will upend the relay database or overwrite the database. If it overwrites, you are safe or else one fine day your mail server will listed in RBL site. ;-)
Thank you for your good advice. I have already set clearopensmtp in cron service.
40 * * * * /home/vpopmail/bin/clearopensmtp 2>&1 > /dev/null
Vpopmail will erase IP in 3 hours by default setting. (configure option --enable-relay-clear-minutes=180)
And, For realize POP before SMTP, I think it necessary to build VPN for security. because user password leak. Not encrypted password !!
Regards, Yasuyuki Toyoda From Japan Tokyo :-P