A few of the schemas used by Gmail are currently not part of the schema.org standard vocabulary. Google has been working with the schema.org community to standardize the mail-related schemas so that other email services and email clients can take full advantage of them. The review process is still in progress.
The schemas may be revised by the time they become standardized. Google is commited to update its support of the schemas as they get revised by schema.org. When that happens, Google will try to make any necessary transition as smooth as possible to all developers using these schemas as well as to their users.
List of Schema.org Proposals
Following is a list of schema.org vocabulary proposals related to schema support in Gmail:
You may follow the discussions about the proposals at the Schema.org Discussion Group.