SPF (Sender Policy Framework) is an open standard that validates your email address. The SPF record is one of the DNS records storing information or regulating the functions of your domain names. SPF is used mainly to prevent email SPAM which comes primarily from forged addresses. That is why mail services use SPF records to check if an email is sent from the mail servers hosting your domain. In an email sent without SPF defined you could write that you send the message from any domain. That is why most email systems don’t accept messages without SPF records or treat them as SPAM. Gmail, Hotmail, Yahoo, AOL, and others require your SPF record and without it, you can’t send emails to their networks. For that reason, you’d better make sure that the email and web hosting services you are using allow the setting of SPF records.
Managing SPF Records
When a mail server receives your message it checks your DNS records for the SPF. If the SPF record is present it checks if the email is sent from the same server that’s in the record. You can place SPF in the TXT record in order to indicate which servers are permitted to send email messages from your domain. For example, when you send emails from firstname.lastname@example.org the SPF records should include yourdomain.com, thus validating the domain.
If you want you can add more of your domains, but keep in mind that the SPF record check has a limit of 10 DNS lookups. The mechanisms used in SPF that need DNS lookups are “include”, “a”, “mx”, “ptr”, and “exists”, part of this list is the “redirect” modifier as well.