Spf

If you run a mail server, you likely want to setup spf and DKIM.

For SPF, edit your DNS records and create a TXT record for your domain that says:

v=spf1 a mx ip4:1.1.1.1 -all

Replace 1.1.1.1 with the IP address of your email server. Once you push the DNS, wait at least an hour and you can test it with:

$ nslookup -type=txt example.com

or

$ dig example.com txt

replacing example.com with your domain name.