Gmail's Delegate and collaborate on email 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 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:firstname.lastname@example.org sender:email@example.com X-Google-Sender-Delegation:firstname.lastname@example.org