Mimestream syncs via the Gmail API, but currently relies on IMAP for push.
IMAP was selected for push because it's a direct device-to-Gmail connection. However, it has some downsides, including:
We plan to provide a new push model that uses the Gmail API, Google Cloud Pub/Sub, a Mimestream event relay service, and the Apple Push Notification service.
We take security and privacy very seriously, and the Mimestream service component will not have your password or access tokens to your Gmail account, and will not have access to any email content. Only push events would flow through the relay service, e.g. the data would basically be "hello@mimestream.com has new email" but this payload would not contain any message content. We will provide a thorough and detailed explanation of this architecture before this is launched.