Mac Mail shows Downloading Messages But Actually Isn't

TLDR: don't worry about it.

It's the same for me. Just checked and it's "Sending 17 of 21 messages" and it will stay like that until I quit and re-start Mail. Those 17 messages will never get sent cos they don't exist. It's been like it ever since I can remember. The numbers are random and can either be downloading or sending. There's nothing in the Outbox. Rebuilding mailboxes doesn't fix it. Synching all accounts doesn't fix it. Connection Doctor says all mailboxes are OK. Network monitor shows no traffic to any of my mail servers. If I disconnect the Mac from the network I don't get error messages that Mail can't send. My iDevice isn't downloading, sending or synching. In the early days when I had fewer accounts I religiously checked my Mac vs Webmail, counting messages to see what was missing. Answer? Nothing. In my case they are phantom messages. Quitting and starting Mail gets rid of them for a while, but they'll come back sooner or later albeit with different x of y numbers.

I deal with it by using side-by-side view and hiding the mailbox list on the LHS so I can't see the status bar. Out of sight, out of mind.

I gave up telling Apple many years ago.

What makes you think that the status message is not actually correct and is just taking a long time to download all of the messages from your accounts? Depending on the number and age of your email accounts there could be many thousands of messages to download when an account is initially added. Depending on if you are using WiFi or Ethernet, and what your Internet speed is, that process can take a very long time. The counts listed can also be misleading as it evaluates multiple accounts and the associated mailboxes.

When I recently added four accounts to Mac Mail on a new Mac, it took many hours to download all messages even connected via Ethernet to Gigabit class Internet. And, that was only the messages. Attachment download settings can be set per-account in Mail's settings. If that is set to download all attachments, then even more time is necessary. When adding an account to Mail I find it best to connect to Ethernet and plug in my laptop, add the account, and then walk away or leave it overnight so the process of downloading decades of messages can complete.

For Side by Side in View on the menu bar tick "Column Layout" and "Side Preview"

The mailboxes on the left appear if you leave the cursor on the LHS of the screen but you can put Favourites at the top. = like this. Any Mailbox can be added to Favourites - including smart mailboxes and also Flags. If you click the far left "Mailboxes" it toggles the mailbox pane at the LHS on and off.