Mythical "sending messages" in Apple Mail

Every couple of days, I get a message in Apple Mail that says, "Sending 8 of 9 messages." Always seems to be "8 of 9."

Only ... I haven't sent ANY messages. If I quit Mail and restart it, message is gone.

I'm using a MacBook Air, 13-inch, M3, 2024 running Sequoia 15.5. Mail is 16.0. I have one Exchange account, one POP, and four IMAP accounts. The option to delay sending is OFF.

I also use SpamSieve 3.1.3