Sender Policy Framework (SPF) records are a method for declaring authorized email senders for a specific domain. SPF records are placed in the domains DNS Zone File and the mail servers use this information to interperet who is an authorized sender and who is not an authorized sender. This is a security measure that ensures only certain domains or IP addresses are allowed to send mail for a specific domain.
It is up to each mail server administrator how strict their SPF record checking is, but the majority of email servers are checking SPF records. This can cause bounced back mail if you do not have an SPF record in the domains DNS zone file or if it is configured incorrectly..
For NanoServers managed email (mx.nanoservers.net), we use the following SPF record:
"v=spf1 a mx ip4:66.148.95.0/24 ~all"
If you are hosting your email elsewhere, you will need to contact them to determine which SPF records are appropriate for your domain.
If you are hosting your own email servers, then you would need to set up your own SPF record. You can use the following wizard to guide you through the process:
http://old.openspf.org/wizard.html
For more information on SPF records please read:
http://www.openspf.org/Introduction
http://www.openspf.org/
