The biggest reasons emails get opened is because the recipient knows and trusts the sender, and they previously opened and got value from this person or company.
These are the results from a survey conducted in December 2006 by Return Path with 2,413 respondents, ages 18-54, in the U.S. and Canada. It found that these factors influenced consumers to open and read e-mail: 
This means that it is more important than ever to build relationships and trust with the people you email to.
When someone signs up and requests information from you, it is up to you to create value each time you email to them.
Write in a personable way, create a conversation, offer valuable information or tips, and keep your messages short and to the point. Only you can judge how frequently you can contact your subscribers and clients. Use common sense and when in doubt, ask.
This poll was first seen in an article, "Why Consumers Open E-mails" by Dr. Ralph F. Wilson, Editor, Web Marketing Today.