Chris Fralic wrote a much needed article on the Art Of The Email Introduction. The first rule, The Ask, is the most important in my book:

“Sometimes it makes sense to just make the introduction when asked, but in most cases I think it’s a best practice to ask for and receive permission before an introduction is made. This makes it a choice for the recipient and doesn’t create an obligation.”

Introduction: 10 Rules For Emailing Busy People, by Chris Fralic