Why limit the scope of your advertising sales? If you put most of your time and energy into generating entry-level leads, then it's time to change things up a bit.
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If you’re thinking of monetizing your email newsletter, then you might want to consider looking for sponsors. Securing a good sponsorship deal can be beneficial for everyone involved. For you, of course, it will mean generating revenue. For your sponsors, it will mean access to ready-made audiences, which in turn will get exposure to new brands, products, or services that give them value.
Email newsletters are a big deal, being the third most common form of content distribution after a company's website and social media. But, while the latter here are great for pulling in new people and upping interest in your business, they alone are unlikely to drive reader retention entirely like email is.