Helping people and organizations turn virtual relationships into real business since 2012. Coauthor of The Virtual Handshake, the 1st book on social media marketing. 7 years as Entrepreneurs Guide at About.com. Started my 1st software company at 20, have worked as an entrepreneur, startup executive, or intrapreneur for 30 years.
Don't delete them: segment them. Move them to another list for their own retention campaign. Send them a series of 3 messages saying that you've noticed that they haven't read anything from you lately, and you want to make sure that they're still interested in receiving information from you. If after that, you still haven't heard from them, delete them. In addition to keeping your costs under control, it will also improve your open/click rate with your sender, which, depending on which service you use, may be important, as it's often taken into consideration if you get a lot of unsubscribe requests or spam reports.