Get into Sender Policy Framework

Sender Policy Framework becomes a standard tool against spam, so you better learn some basics about SPF and try to set up your record. Especially if you are proud owner of a big newsletter but wondering why your delivery rate is decreasing slowly but surely get started with theses sites:
http://spf.pobox.com
http://spftools.infinitepenguins.net
http://www.dnsstuff.com/pages/spf.htm