Email marketing is a medium with high ROI due to the volume of targets reached. However, there is also a lot of spill since a large number of messages conveyed over email marketing goes undelivered. Here are the percent of undelivered emails for the different American ISPs:

Gmail : 23%
Hotmail : 20%
MSN : 20%
Comcast : 17%
AOL : 16%
Yahoo : 15%
NetZero : 14%
BellSouth : 14%
Roadrunner : 12%
USA.net : 11%
Cox : 8%

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