Ten cents per thousand emails

The instinct is to send campaigns through the mail you already have, because it is already there and it already works.

Three things go wrong. There are sending caps you will hit. There is a real risk of the account being suspended for bulk sending, which takes your normal mail down with it. And there is reputation: every bounce and every complaint attaches to the domain you use for actual business correspondence.

A dedicated sending service costs roughly a dime per thousand messages, which is close enough to nothing that cost is not the deciding factor. What you are actually buying is separation — the campaign traffic lives somewhere it cannot damage the address you need people to receive.