Mail app icon on MacOS dock displaying incorrect unread message count

I have gone through the troubleshooting steps mentioned therein here: search

But, to no avail.

Currently, it show the number 2 within the red dot (yet I have no new messages in the inbox or junk folder). I have to struggle to read that number though, as white on red is a tough combination. Yes, I have my eyeglasses on, but still, it is a pain to distinguish. Sometimes, I suspect developers all have 20/20 vision.

[Re-Titled by Moderator]

Original Title: Why does the Mail app icon on the MacOS Dock showing a number while there is no new messsages?