Apple Mail - Message stuck in Outbox and Keeps Flashing/Popping Up and Closing

I will send a message from Apple Mail and it makes the send noise, but then I see the message is in my outbox and the message keeps popping open for half a second and closing, over and over and over again. The only solution is to delete the message from the outbox and try resending it.

It's extremely annoying. I see someone asked this same question 3 years ago and had no response.

I am using an iMac 27", 2020 - Intel Core i5 running Sonoma 14.4.1 (but this same issue has happened with earlier versions of the OS for years).