I found this topic while searching for an explanation of why Mail's left Mailboxes panel was showing "Sending 30 of 30 Messages" when all messages had been sent. I just sent a test message to myself, which was instantly delivered ... and the count is now 31.
After quitting and relaunching Mail, the footer of the Mailboxes panel reported "Connecting" during startup and then disappeared. When I sent another test message to myself, the footer appeared for just a moment, reporting "Sending Message" and then "Downloading Messages - 1 new message."
I'm running Mail 16.0 on Sonoma 14.4.