On Ubuntu server, how can I receive "new mail" notifications on the terminal while logged in?

I connect to my Ubuntu 20.04.2 LTS server via SSH, and the "You have new mail" notification works properly when logging in, however, if I receive mail while already logged in, I don't get any kind of notification.

I remember this working properly on other Linux systems in the distant past and I can find references online regarding it working for other people but no instructions on how to make it work.

If I receive mail while logged in I'd like the terminal to automatically display a notification message.

Based on searches for possible solutions, I checked the value of the "$MAILCHECK" environment variable and found that it's already set to 60 seconds, so I haven't made any adjustments to it.

I found a package named "mailcheck" that was not installed so I installed it, however, nothing visibly changed. Manually running the "mailcheck" command never outputs anything even when I know I have mail in the mailbox.

I eventually found that there's an environment variable $MAIL that's supposed to be set but (for some reason) wasn't.

So I set it via my .bashrc:

MAIL=/var/mail/username

This resolved the issue.

The "mailcheck" package ended up being irrelevant so I purged it.

The following documents were useful in solving this:

https://www.shell-tips.com/bash/environment-variables/#the-mail-variables-mail-mailpath-and-mailcheck

https://docstore.mik.ua/orelly/unix/upt/ch21_08.htm