Hi! Thanks for the kind words.
I hate to disappoint, but Mailpile isn’t a cloud service, nor is it a tool for building cloud services. It’s a single-user e-mail application. You may as well ask whether Thunderbird comes with an API to create accounts with.
The answer is, both for Mailpile and Thunderbird, no.
And to further clarify: we consider online e-mail services where some random company holds all your data on your behalf, to be a privacy problem, and we would rather our code were not used to create more of them. That is literally why we created Mailpile, and that is why Mailpile is AGPLv3 licensed. You might want to double and triple check whether your intentions can even remotely comply with our license.