Has this ever happened to you? You attend an event and give your business card to some of the people you meet. A few days, or weeks, later you receive an e-newsletter from them. The problem is they never asked whether you'd like to receive it. They just signed you up!
Happens all the time, doesn't it? Don't like it much, do you? Nope. Because you weren't asked. It's somewhat intrusive. They didn't ask whether you were interested in receiving their newsletter - they didn't ask for your permission. They made an assumption. They thought it was a clever sales tactic.
This is a terribly bad practice because it's really salesy and not respectful. They've invaded your inbox and put you in the uncomfortable position of opting out - and potentially hurting their feelings.
Prospecting, networking, and sales are all about respect and relationships. It's pretty hard to build a relationship when you don't exhibit respect for the other person.
So, remember to ask if they'd like to receive your newsletter. And if you get back to the office and can't remember who said yes, ask all of them. It's the courteous thing to do. And peopel will liek you for it!