Adding an SPF TXT-record to your domain's DNS settings helps fighting spam. However, often there is a webserver that also wants to send mail using your domain as sender. Google's suggested SPF setting is not sufficient for this. Add (or replace if it already exists) this DNS TXT-record to your domain's DNS:
v=spf1 a include:_spf.google.com ~all
The only difference with the default value suggested by Google is the extra "a" after "v=spf1". Read Google's original instructions here. This extra "a" means that all IPs in the A-records of the domain are allowed to send mail. That should include your web server because your web server its IP is set in an A-record.