There are three things that you can do to decrease the likelihood of spammers spoofing your from address:
- Whitelist: Address, domain names, or IP addresses listed here will bypass spam checks. It is not advised to place your own domains or email addresses in the list. Spammers use spoofing as a method of trying to bypass spam filters and IMail checks the incoming MAIL FROM: address against the list. If your domains or email addresses are spoofed, and you have them in the Whitelist, these messages will bypass spam filters and be delivered to the end users.
- Authentication: Make sure 'No Mail Relay' is set in the SMTP settings. As long as users are Authenticated, they will not be filtered by the Antispam settings.
- SPF (Sender Policy Framework): Open SPF has all the information on SPF, as well as a wizard to assist in setting up your records.