Comment by lost-theory

Agreed. That's why it might make sense to fight NIH on the general problem of sending emails, but to give into NIH when deciding to use an "external service to handle email drip campaigns" vs. building it in-house. That type of problem is much more specific to your needs and attempting to outsource it could cost you more time and complexity than it saves.

Using an ESP for delivering email is a no brainer in most cases.