I found myself in a discussion about email list building today. There was an idea proposed that had the group divided:

Background: We want to send timely and useful content, as well as a marketing message for our services, to people that attend specific trade shows (trade fairs).
Idea: Purchase a list of names of attendees to that event from the previous year. Send them an email with relevant content and an offer for our services to help prepare them for this year's show. Also include a prominent opt in / opt out for future communications related to this event or other events related to their industry.

We would obviously honor the opt-outs, and only continue to send messages to those who want to hear from us.

Thanks in advance for your responses.