SPF

We recommend that you create a Sender Policy Framework (SPF) record for your domain. An SPF record is a type of DNS record that identifies which mail servers are permitted to send email on behalf of your domain.

The purpose of an SPF record is to prevent spammers from sending messages with forged From addresses at your domain. Recipients can refer to the SPF record to determine whether a message purporting to be from your domain comes from an authorized mail server.

Ours looks like this:
v=spf1 mx a include:registrar-servers.com include:_spf.google.com ~all

If you just want to use the MX records and A records to control who is valid, then "v=spf1 mx a -all" is the text record that works. This wizard can help. Once you're done, you can check on things with nslookup -query=mx

Resources

1. Configure SPF records to work with Google Apps (Google)
2. How to use an SPF record to prevent spoofing and improve email reliabiltiy (Digital Ocean)

Sender Policy Framework

Domain Name System. An Named Host to IP Address lookup system