A business card exchange is not permission to add someone to your e-mail list. A quality opt-in (confirmed request to send information) or double opt-in list is the best way to go. Think quality over quantity with e-mail marketing. Developing a list is not a sprint, it’s a marathon.
While it may be tempting to grow the list and buy e-mail lists from other providers/partners, that method requires extensive research and may require professional advice to ensure the integrity of the list. Marketers who buy lists can never truly know how the names were acquired, the quality of the e-mail addresses, or how many times the names have been sold to other marketers.
Most third-party e-mail management companies will only allow opt-in lists, and will cancel relationships with organizations that add names without permission attached to them.