Question: When sending a forms to a client for signature. It was sent from the old email address. Can you let me know which setting default or set the outgoing email address?

Answer: Check first your "Email Settings" and make sure email address is updated. You should disconnect/connect your email server, learn more HERE

Reason for bounced messages from Microsoft documentation:

Solution: User should check the email address and outgoing mail server, and should purchase the license. As documentation says the email address is probably trial.