Apple Mail on Mac stuck "sending messages"
Mac OSX 10.15.2 Catalina, Apple Mail client 13.0. Message stuck in outbox, "Sending Message" message down near the bottom. Forever. Connectivity, functionality, and credentials of email accounts all verified. Incoming not affected. This is impacting messages "sent" (not sent) via an Exchange account and an imap account.
Outgoing server details seemingly in order.
Help.
Mac Pro, macOS 10.15