When attending or participating at trade shows and conferences, you and your staff will collect a large number of business cards. When someone hands you their business card, they are giving you permission to make a follow-up call and nothing more.
A business card does not equal carte blanche permission to subscribe them to your opt-in email marketing list. It does, however, open the door for the possibility. So follow up on that lead, and ask if you can add them to your email marketing list. Don’t forget to share a few benefits and let them know the frequency of your emails.
Even better, if you have a booth at a trade show, have visitors (your potential customers) sign up for your etips/email newsletter on the spot.