Most of scanners should support SMTP auth, so as we setup ALL scanners and multifunction devices with our Zimbra server, we simply create user for scanner, like
scanner@company.com and setup scanner to use Zimbra server as outgoing server, and use SMTP authentication, username "scanner@company.com" and password "some_password".
Works all the time.
...except in situation, where there is some local mail server. Then we setup local mail server to accept mail from scanner.
....or use your ISP SMTP server to send scans like regular mail.