To be sure that a newsletter with innocent words and formatting get not trapped in people’s or ISP’s spam filters, it’s useful to check it with an appropriated tool. Site Build It offers a free SpamCheck tool by form on the web or by e-mail.
I use the free MailingCheck software (version 104) from eDisplay to analyse my newsletters before sending them to the recipients. The test-email file has to be in .eml or .htm format. MailingCheck, as most spam check tools, use the open-source Apache SpamAssassin Project.
Another tool used by ISP’s to prevent spam are blacklists. A useful guide about this topic is available at the SpamLinks website. Blacklists or blocklists are lists of IP addresses, domain names, email addresses or content of the headers or the body, or some combination of these different types, that can be used to help identify spam. A special subset of IP address and domain name lists exist which can be queried using DNS, which are called DNS Blackhole Lists or DNSBLs. Blacklists can be unverified and cause “collateral damage”; their criteria for listing may not be clear.
I use the free Anti-Spam Blacklist Thing software from BlacklistPatrol/SpamButcher.
Blacklist Patrol offers also a paid service to proactively monitor major anti-spam blacklists to see if an email server has been listed on any of them.
More informations about spam and blacklists can be found on wikipedia.