Google has released an updated Gmail API. The update includes a number of new endpoints that enable a host of new features: filters, forwarding addresses and auto-forwarding, IMAP and POP settings, send-as aliases, signatures, and vacation responder. Many of the features have been long-requested, but never available in any Google API (e.g. out-of-office auto-replies). The updated Gmail API functionally replaces the Google Email Settings API. Accordingly, Google announced the deprecation of the Email Settings API.
The majority of the new endpoints are immediately available with any Google Apps or Gmail account. However, features that Google deems "sensitive" (modifying send-as aliases, forwarding, etc.) require service accounts with domain-wide authority. For more details regarding such requirements, visit the reference docs.
Methods available for the filters include create, delete, get and list of filters. Filters available include id, criteria, from, to, subject, query, attachment, chats, size, and size comparison. Developers can delete or retrieve specified forwarding addresses, or create and list forwarding addresses for specific accounts. The send-as aliases feature corresponds to the "Send Mail As" feature in the Google web interface. The alias can be the primary login address associated with an account or a custom "from" address. The signature is a optional HTML signature included in messages. The vacation responder is an auto-generated response when set, and developers can retrieve or update such responses through the API.
The Gmail API provides a number of features historically limited to enterprise-grade email software. Google continues to move into the enterprise space and such API features should enhance its ability to do so. While Google has deprecated the Email Settings API, the API will remain available until July 7, 2017. Google has published a migration guide for those needing to transition from the Email Settings API to the Gmail API. Further, check out the gmail-api tag on Stack Overflow to troubleshoot specific issues.