Which mail provider, with an API, do you use for your campaigns?

For a long time we used Mailchimp for our campaigns. We would upload and segment/group the users via the Mailchimp API. The newsletters would be hand-crafted and sent. Over time Mailchimp became cumbersome - their interface has become so dumbed down if personally feels unusable. Also, the $$ seemed misaligned with the value.

We started looking around for a new mail provider...a great API + GUI for creating newsletters + less expensive than Mailchimp.

We settled on Mailerlite. So far like about 90% of the offering.

What mail provider have you used, and like?