Back when sending emails was about 1 cent per email, my company wrote our own campaign email software to save tons of money. Each large email provider has its o

Back when sending emails was about 1 cent per email, my company wrote our own campaign email software to save tons of money. Each large email provider has its own rules about sending rates, backing off, soft bounces and hard bounces. Not getting your IP address blocked for breaking a ruleset that you don't know about is the hardest thing. A few year later MailChimp was becoming more popular and their prices where dropping; we switched almost everything over immediately and life was much easier.