The specifications for SPF (Sender Policy Framework) require only one TXT record. If you already have a TXT record with SPF information set up for your domain name, you will need to customize it following specific syntax rules. Here are some basic mechanisms that may come in handy.
1. Allow any other domains/subdomains to send mail on behalf of your domain.
In order to add other domain names or hostnames that may send or relay mail for your domain, use the mechanism ‘a:’ and separate the list with spaces, for example:
"v=spf1 a:example.com a:sub.example.net -all" 
In other words, if the example.com domain resolves into 1.2.3.4 and the subdomain sub.example.net into 5.6.7.8, it means that these IP addresses are allowed to send mail for your domain name.
2. Allow specific IP addresses to send mail on behalf of your domain.
If you need to authorize a list of IP addresses that can send mail for your domain name, use the mechanism ‘ip4’ and ‘ip6’ respectively and separate them with spaces, for example:
“v=spf1 ip4:64.233.167.99 ip6:1080::8:800:200C:417A -all" 
IP addresses of the same class may be published with a slash, i.e., 192.168.1.14/32, like "v=spf1 ip4:192.168.1.0/38 ip6:1080::8:800:200C:417A/96 -all" 
3. Allow a third-party service to handle mail for your domain.
If you send mail through a different domain name, e.g., your mail service company, you may pull in its SPF record using the mechanism ‘include’, for example:
"v=spf1 include:spf.mandrillapp.com include:spf.google.com -all" 
That's it!
If you have any questions, feel free to contact our Support Team.