[Gmail's Delegate and collaborate on email][1] allows adding delegates who are able to access someone else's mail box and read, delete and send from there. It states:

> You can add up to 10 delegates. If you're using Gmail through your
> work, school, or other organization, you can add up to 1000 delegates
> within your organization.

Is there a way to use [search operators][2] to find only messages that were sent by a given delegate?

Examples that don't work (the latter two are what is actually shown in the full headers):

    sent_by:[email protected]
    sender:[email protected]
    X-Google-Sender-Delegation:[email protected]

  [1]: https://support.google.com/mail/answer/138350
  [2]: https://support.google.com/mail/answer/7190