There are several dissenters to our statement in Tip #8: Never rent an e-mail list. Note that e-mail rules are different than snail mail. No one cares about junk mail, but they do object strongly to receiving "spam". Privacy is a huge issue for internet users.
So, if you feel you want to rent such a list, make sure it truly is "opt-in," specifically permitting you to send e-mails to the list. Carefully check the actual wording of the opt-in statement. And of course make sure that the people on the e-mail list are appropriate potential attendees for your show.
But the best way to use an outside e-mail list is to have the owner send the message on your behalf. For example, if the list is owned by a trade magazine, have the editor send an e-mail inviting all Widget News subscribers to her dynamite panel session on "The Internet in Widget Production." Offer free admission to the session (and show) by clicking on a special hyperlink just for Widget News subscribers. This could be part of the publication's regular weekly (or possibly daily) internet newsletter.