In 2002 the European Union (EU) introduced the Directive on Privacy and Electronic Communications. Article 13 of the Directive prohibits the use of personal email addresses for marketing purposes. The Directive establishes the opt-in regime, where unsolicited emails may be sent only with prior agreement of the recipient; this does not apply to business email addresses.
Another collection method happens when list vendors buy emails lists from industry trade shows (or other events) where people give their info during the registration process. This is not the same thing as folks who signed up with you, directly, at your trade show booth! This is where list vendors purchase the entire registration list, from the trade show, itself.
Julie Ewald recommends moving away from sending generic content to your whole email list via e-blast entirely. With a little elbow grease, you can utilize segmentation and start sending off tailored messages to each of your lists with content that’s relevant, valuable, and designed to resonate with the kind of recipients that make up each segment.