IMAP API is a self hosted application for accessing IMAP accounts over REST API. IMAP API daemon connects to user's IMAP account, translates API requests to IMAP and also sends webhooks for changes like new or deleted emails.
IMAP API is self hosted and lightweight daemon application. It does not store any emails, except for the minimal amount of data required to keep mailbox state in sync.
Source and details: IMAP API in Github
IMAP API is part of the Nodemailer project.