Create your a SPF record

Adding an SPF record to your DNS zone file is the best way to stop spammers from spoofing your domain.  You set which mail servers are authorized to send email on behalf of the domain in the DNS record. In addition, an SPF Record will reduce the number of legitimate e-mail messages that are flagged as spam or bounced back by your recipients' mail servers. The SPF record is not 100% effective, unfortunately, because not all mail providers check for it.

An example SPF record looks like this:
v=spf1 ~all
In the example:
v=spf1 is an SPF record
include: means email can only come from these sources
~all considers any other email as a soft fail