Sender Policy Framework / SPF
A sender policy framework is a way to sign mail to authenticate the sender domain, a bit like DKIM / ADSP. DNS records are added to the zone file in order to achieve this.
An easy way to implement this is to add the following to your DNS record:
TXT "v=spf1 mx -all"
You always put the -all at the end of the record, as that's where the check stops processing and it tells the checker no more conditions will be met.
You do have to make sure that the DNS record has a valid MX record and that the url in the MX also has a valid A record in this case :)