It would be interesting if you could define a restricted-API for email, with just enough functionality to provide your service. The issues right now is that th

It would be interesting if you could define a restricted-API for email, with just enough functionality to provide your service. The issues right now is that there is no fine-grained way to grant permission to ones email, it's all or nothing. Of course, it might take a long time for email providers to offer such restricted-API access, if ever. As a stop gap, you could offer an open-source daemon your users could self-host. It would act as a gateway, granting you limited access to their email, without you needing their password or cart-blanche permissions.