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How to Avoid Email Fatigue
http://cdemails.com/articles/5241/1/How-to-Avoid-Email-Fatigue/Page1.html
iMedia Connection
iMedia Connection is published by iMedia Communications, Inc., a trade publisher and event producer serving interactive media and marketing industries. The company was founded in September of 2001 and is a subsidiary of dmg world media. 
By iMedia Connection
Published on 04/21/2009
 
by Dave Lewis - Email marketing continues to be the most cost-effective strategy for building and maintaining relationships with customers. According to the Direct Marketing Association, email returned $45.06 for every dollar spent on it in 2008. ROI is undoubtedly an important consideration, but dialing up the frequency of emails may be the wrong way to go if your objective is to retain subscribers and optimize deliverability and effectiveness.