scope=https://mail.google.com/的OAuth2。之前的答案已不再准确。最近发布的Gmail API允许使用“只读”范围等其他范围，请在此处找到：https://developers.google.com/gmail/api/auth/scopes。
“只读”范围是：https://www.googleapis.com/auth/gmail.readonly
https://www.googleapis.com/auth/gmail.readonly
Read all resources and their metadata—no write operations.
https://www.googleapis.com/auth/gmail.compose
Create, read, update, and delete drafts. Send messages and drafts.
https://www.googleapis.com/auth/gmail.send
Send messages only. No read or modify privileges on mailbox.
https://www.googleapis.com/auth/gmail.insert
Insert and import messages only.
https://www.googleapis.com/auth/gmail.labels
Create, read, update, and delete labels only.
https://www.googleapis.com/auth/gmail.modify
All read/write operations except immediate, permanent deletion of threads and messages, bypassing Trash.
https://www.googleapis.com/auth/gmail.metadata
Read resources metadata including labels, history records, and email message headers, but not the message body or attachments.
https://www.googleapis.com/auth/gmail.settings.basic
Manage basic mail settings.
https://www.googleapis.com/auth/gmail.settings.sharing
Manage sensitive mail settings, including forwarding rules and aliases.
Note: Operations guarded by this scope are restricted to administrative use only. They are only available to G Suite customers using a service account with domain-wide delegation.
https://mail.google.com/
Full access to the account, including permanent deletion of threads and messages. This scope should only be requested if your application needs to immediately and permanently delete threads and messages, bypassing Trash; all other actions can be performed with less permissive scopes.
GMail API文档指出，只有一个作用域可以访问所有内容。
此外，还有一个名为GMail Inbox Feed的功能，可能是您需要的，但它仅适用于Google应用程序域。