u can get emails from isp email server with fetchmail. fetcmail can inject emails to local server. and u can set some alises for this account. all of your clients can access these emails from local email server.
u can add a cron job. so it will run fetcmail to get emails from isp email server. like every 5 minutes.
actually u use qmail with an antivirus. fetchmail gets emails from my isp and my clients get their emails from local server. and my local qmail server checks viruses. so viruses cant brother my clients.
here is an example. create a file like this.
poll mail.isp.com with proto pop3
user "username" there with password "user_pass" is "user@local_email_server" here
u must change file permissions to 710
chmod 710 /etc/getemail
add a cron job like this:
*/5 * * * * fetchmail -f /etc/getmail
i dont use sendmail. as i remember, /etc/aliases must be edited add some aliases.
i hope these help u