A sender policy framework(aka SPF) record is a DNS TXT record which declare all the authorized server for sending email.
For example, server A sends an email with its IP address "A_ipaddress" and return-path get-bounced@my.domain.
Server B just received the email, then it searches the SPF record of the domain "my.domain".
If server B find a IP address record "A_ipaddress" in the SPF record, then the SPF check passes, or this email will be rejected or marked as spam.
A text record needs a name and a content. Enter the domain name into the name field, if your have two or more email domain names, you can't just insert two or more text records, that will make your SPF record failed. For example, we have two domain names used for email, you should insert one SPF record and include another domain name in the content like this: include:mail.my.domain, then insert another SPF record.
A SPF record's content will be like this:
v=spf1 ip4:192.168.0.1 ip6:2E11:0000:0000:0001:EE01:A221:F111:EE00 a mx include:mail.my.domain -all
v: indicates the SPF version;
ip4: Authorize mail servers by IPv4 address or address range;
ip6: Authorize mail servers by IPv6 address or address range;
a: Authorize mail servers by domain name;
mx: Authorize one or more mail servers by domain MX record;
include: add another domain name's record;
~all: receiving servers typically accept messages from senders that aren't in your SPF record, but mark them as suspicious;
-all: receiving servers may reject messages from senders that aren't in your SPF record.