Sender Policy Framework (SPF)

Sender Policy Framework is used by email vendors (hotmail, gmail etc) to try and spot spam. Basically it is a DNS record that lists the servers that can send mail for that particular domain.

I cannot confess to knowing everything there is to know about SPF records, but here is an example TXT that says you can send emails from the IP specified. You can include multiple IPs.

"v=spf1 ip4:123.123.123.123 -all"

This example says you can send email from domain.com. Again, you can include multiple domains.

"v=spf1 include:domain.com -all"

As I say, I am not expert, but this should get you started.