SPF record

The spf record is in the zone file

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To publish SPF, a domain has to add only one line to its zone file. That line is a TXT record, and you can publish it today. Let's see what the TXT record looks like.
SPF by Example
Suppose example.com wants to publish SPF. It expects MTAs everywhere to read its SPF record and use it to reject forgery attempts. It hopes SPF reduces the volume of joe-job bounces and bogus abuse reports. So it adds the following line to its zone file:
example.com. IN TXT "v=spf1 a mx ptr -all"
The v=spf1 version string identifies this as an SPF record. The -all means reject all mail by default. Domains that don't send any mail, such as altavista.com, can get by with simply v=spf1 -all. But if the domain does send mail, it declares mechanisms that describe how legitimate mail should look. Mechanisms go in the middle, before -all. The first mechanism to match provides a result for the SPF query. -all always matches and so belongs at the end.
Basic SPF
A: the A mechanism means the IP address of example.com is permitted to send mail from example.com. If you want to say the IP address of some-other.com is permitted, you can say a:some-other.com. You can use as many A mechanisms as you want.
MX: the MX mechanism means the MX servers for example.com all are permitted to send mail from example.com. If you want to say the MX servers for some-other.com are permitted, you can say mx:some-other.com. You can use as many MX mechanisms as you want.
PTR: the PTR mechanism says if a host has a PTR record that ends in example.com, it is permitted to send mail from example.com. This would be a good choice for Yahoo, whose mail server names all end in yahoo.com. It would be a bad choice for a broadband provider like Comcast. If you want to say servers whose names end in some-other.com are permitted to send mail from example.com, you can say ptr:some-other.com. You can use as many PTR mechanisms as you want.
IP4: to say the class C network of 192.0.2.0 is permitted to send mail from example.com, you would write ip4:192.0.2.0/24.
Mechanisms are interpreted left-to-right. Using v=spf1 a mx ptr -all first would check whether the connecting client was found in the A record for the domain or, failing that, in its list of MX servers. Then the MTA would check to see whether the hostname of the client matched the domain. If none of the mechanisms matched, -all would be evaluated, the result would be fail and the MTA would be justified in rejecting the mail.
A, MX, PTR and IP4 are enough for the overwhelming majority of domains. The setup wizard at spf.pobox.com/wizard.html can help you configure SPF for your domain. But if your situation is complex, you can use the mechanisms described in the “Advanced SPF” sidebar.
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MAILING PROBLEM

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The above specifes that this is an SPF record for mydomain.com:

v=spf1 - a required header that indicates this is an spf record
a - the A record for mydomain.com is allowed to send email
mx - the MX record for mydomain.com is allowed to send email

~all - all other mailservers trying to send mail will return a code of "softfail". The mail will still go through, however it will be a suspect message

If you wished to allow another server to send mail through that had the hostname of mail2.mydomain.com, you would modify the record thusly:

mydomain.com IN TXT "v=spf1 a mx a:mail2.mydomain.com ~all"

Also, if you wish to give a hard fail, and only allow those servers to send mail, the record would look like:

mydomain.com IN TXT "v=spf1 a mx a:mail2.mydomain.com -all"
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