Can I post promotional messages on public message boards or mailing lists?
No. For example, you can't post a Usenet newsgroup message saying "Visit www.example.com for the finest fruit pies!", even if the group is full of people talking about fruit pies. Newsgroups, website message boards, and mailing lists are places for people to exchange ideas, not advertising (unless the group is of the rare type that explicitly encourages announcements of new websites, services or products).
Mentioning your website in a public place may be okay if you're contributing to a discussion that is initiated by other people (for example, if someone asks "Does anyone know where I can buy fruit pies?"). However, many people will still consider such self-promotional responses inappropriate. If in doubt, first ask the other group members whether commercial responses are considered acceptable.