Our business found Mailgun to be a great solution when we anticipated clients would stick under 10k/mo and not need a card on file. With Stripe, every time they pay their cut, they've had a sale. Every time they mail-out through MailChimp, it's to a list of customers.
We are usually using Mailgun to send transactional emails. I can't promise a client that they won't get a flurry of junk signups or password reset mailouts that hit their credit card.
The bottom 50% of the market is heading to self-serve site-builders and it's savage for a small web business.