“Message Undeliverable”

2018-05-16 22:22:32

As of this morning one of my gmail addresses is receiving email but is not sending. Every email I've attempted to send or respond to comes back with the message "Undeliverable" and gets stuck in my outbox. I've rebooted. I've deleted the account and re-installed. For a brief moment it seemed to be working properly again. But that did not last long, and it is not working again.

My other gmail account is functioning normally.

Any thoughts?