Marketing Experiments found that when emails focus on just one service they significantly outperformed the email which tried to describe and pre-sell four separate services. You may want to test sending out a single offer email versus a multi-offer email.
The only problem I have is that if a company has multiple product lines, the notion of frequency comes into play. Do I really want to annoy my subscribers by sending multiple single offer emails?
Testing Emails Offering Services, iMedia Connection
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