Ask Before You Intro
One of the most time wasting moments in a persons professional life is when you open your email and get an unsolicited, out of the blue message like this from someone you know:
Fred is CC'd on this email and is working on a new startup. You should connect soon!
Lets meet up for a drink sometime! Sent from Hawaii![A]
Why is this email bad etiquette?
Blind Intros Are Bad For Everyone Involved
The email intro was unsolicited aka a "blind intro". In other words, Ivan did not warn Becky he was going to intro her to Fred. This implies:
Ivan is implicitly stating that he does not value Becky's time enough to ask in advance if she wanted an intro. He is assuming she will make time for Fred, even if she is really busy or completely irrelevant to the request. No context is provided on why she would want to talk to Fred, what Fred wants from her etc., and no opportunity is given to decline the intro before it is made.
Ivan is trying to show Fred that Becky is part of his network and will help him with random requests. It allows Ivan to "add value" to Fred by taking advantage of Becky's time. Many VCs or angels make these blind intros a lot to demonstrate value to an entrepreneur they are courting.
Ivan is making Becky the "bad guy" if she does not have time to meet with Fred post-intro. Becky is in a tough spot. She can reply to Fred saying she is too busy, but that will make her look bad to both Fred and Ivan. Or, she can ignore Fred or treat him poorly when they do talk, which makes both her and Ivan look bad. (All else being equally, you should just reply saying you are too busy if this happens to you if you need to make the trade off).
Ivan is setting Fred up for a bad experience with Becky. This also reflects badly on Ivan in Fred's eyes in that:
Becky may never reply to Fred. Fred may waste time re-pinging Becky over and over.
She may not be the right person for Fred to talk to.
She has no context on why Fred wants to talk. It is possible she could have directed Fred to someone more helpful, but Ivan never gave her the chance. Does Fred want to talk about fundraising? His product? A partnership with Becky's company? Does he need an intro to someone else Becky knows? What is the purpose of the intro?
Ivan is burning useful social capital. Becky is less likely to help a friend or co-workers of Ivan who really needs help if Ivan burns up her bandwidth/time inappropriately with random blind intros.
The Right Way To Do It
Before doing an introduction, you should ask both sides if they actually want one. You should set context and provide someone with an "out" in case they are not interested or able to make the connection (e.g. if they have a conflict of interest, a lack of relevance, or the like). This sets you up better for the future - if you tell someone you really need them to talk with one of your contacts, they will believe you and make the time.
Example good email:
I met Fred at a hack-a-thon last week. He is working on a really cool iphone app, Dogatune, that lets you autotune your dog's barks. I have been using it the last 2 weeks and love it. Before this, Fred was at Facebook working on the newsfeed and Ron Conway / SVAngel has already offered to fund his company if he raises money .
I know that you just raised a round of funding and really like working with SVAngel, so he was hoping to chat with you about your own experiences working with them .
Is it OK if I do an intro? I know you have been really busy lately with your own startup, so understand if you don't have time to chat with Fred .
 Context on how you know the person and for how long. How important of a connection is this for you? Is this one of your closest friends, a former boss or employee, or some random person you don't really know or care about? This helps Becky prioritize the request based on how important it is to you.
 Context on what Fred is working on. This helps Becky filter whether she can be of help.
 Social proof is always helpful. It provides context on how legit Fred is (or is not).
 Specific context on what Fred needs help with. This is key. It allows Becky to assess if she can actually be helpful here or not as well as the purpose of the potential intro. Notice that Ivan also explained why/how Becky is relevant to the request (she worked with SVAngel before).
 Ask explicitly if you can do an intro. You will be surprised by how many people say no if given the choice. They will thank you for it, and take your future intros much more seriously.
 An easy out is provided. It shows you value the person's time.
How To Respond To A Blind Intro
Politely ask the person who did the blind intro to stop making them to you. This can be phrased nicely - e.g. "I am really busy and want to help people in your network. If you intro me to someone and I don't have time to reply or I am irrelevant to, it may reflect poorly on both of us. Can you please ask me before making an intro in case I can direct the person to someone who may be more useful or relevant?"
Ask for context from either the introducer, or the introduced. E.g. "Fred, nice to meet you. Can you please provide context on what you want to discuss?". I have found a pretty large number of blind intros either (a) are not relevant to me (I can say as much, which means it is a polite decline) or (b) unexpected for the other person too (who has just been put in the same spot as me! I.e. a double blind intro).
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[A] For some reason, I have noticed people who do lots of blind intros also tend to travel a lot, often on exotic boondoggles.
A few people mentioned their heuristic for intros was to ask themselves if both sides of the introduction would derive real benefit from it. If so, they would make the introduction.