Sender Policy Framework (SPF) is an open standard that aims to help prevent spam, and stop legitimate email from being tagged as such. While the specifications can take some time to understand, there are some web-based tools to help you make creating your SPF records a little easier than by hand, but I still find them to be a bit misleading. According to the SPF FAQ the simplest way to create your SPF record is to simply list the IP addresses of all servers that mail goes out through, e.g.:
v=spf1 ip4:192.168.0.10 ip4:192.168.0.12 ~all
and just add more "ip4" strings for each address. Then just paste it into a TXT record in your domain's DNS and you're good to go!