Maybe you keep misspelling Viagra. Seriously, some spell checkers get suspicious when they see "cute" spellings or unusual characters inserted into words, so it's best to avoid these. When possible, use your domain's outgoing mail server. An outgoing server that doesn't match your domain looks suspicious unless it's a well-known one, like Gmail or Yahoo. Avoid links to graphics on the web. Go to www.robtex.com to see if your IP address or host name has been blacklisted. It happens, even to the innocent.
(This is an excerpt from "25 Questions, 25 Answers," an upcoming feature from our 25th anniversary issue in March 2008. Please feel free to add your own suggestions.)