Alexis Kold, a startup founder and investor, writes about fundraising, customer networks, and how to write introduction emails.
HN links his post on writing a forwardable introduction email as a good guide alongside similar posts on the topic.
Here are some other great posts about this:
http://also.roybahat.com/post/92544681186/introductions-and-the-forward-intro-email
https://alexiskold.net/2015/06/24/how-to-write-a-forwardable-introduction-email/
http://avc.com/2009/11/the-double-optin-introduction/