As others have said, your pricing is unpredictable and hard to compute. As I’m evaluating you with competitors, I’m not going to invest the time. In my experience, when a company makes it hard to know what you’re actually going to end up paying, you usually end up paying more than you want (see: Verizon).
From a business perspective, it seems to me your value proposition is that you are striving to be the cheapest email provider. I think you should consider the kind of customer this attracts. The sort of person who thinks a few dollars a month per email address is too expensive is going to be high maintenance. If you want this to be sustainable, I think you need a different angle. Consider that all these other email providers basically charge around the same amount. That isn’t an accident. I don’t know the email business, but I assume it needs a certain margin to actually be sustainable and these guys have landed in the right ballpark.
If you’re looking for business customers, then your pricing is basically rounding error compared to the other guys. It’s not materially different enough to matter for a business of any reasonable size to be worth having as a customer. In fact, the lack of predictability is a major deterrent. Businesses need to create budgets and every variable cost adds to the complexity of the forecast.