Gmail Security – How to Read Spam Mail Safely

While the list of criteria used by GMail's spam filter is a trade secret, most spam filters are quite similar.

The list of spam criteria used by a very popular Spam Assassin spam filter is made public, together with weights.

Some of the most important ones, cherry-picked by me:

• Character set indicates a foreign language
• Body contains a ROT13-encoded email address
• Message body has many words used only once
• Message body mentions many internet domains
• Received via a relay in Spamhaus XBL
• Received via a relay in list.dsbl.org
• Subject contains a gappy version of 'valium'
• Subject contains a gappy version of 'viagra'
• Attempts to disguise the word 'viagra'
• Relay HELO'd using suspicious hostname (IP addr 1)

And so on.

There are some services that allow you to test your message for compliance with the spam filters and see what exactly is wrong with it — http://spamcheck.sitesell.com/ is one of them.

Jeff Atwood had a really good post on things to do to help your email get through. This made a huge difference for the company I work for. Some highlights:

1. Make sure the computer sending the email has a Reverse PTR record.
2. Configure DomainKeys Identified Mail in your DNS and code.
3. Set up a SenderID record in your DNS.