Scroll bar missing in mail app ?

Take a look at what you have set in System Preferences -- General pane under the "Show scroll bars" setting -- try setting "Always" or one of the others.

Good luck...

Old problem or a new problem, what has changed?

Can you use two finger swipe up and down?

Have you quit Mail and relaunched?

Have you rebooted the machine?

You do not say what exact Mac /model /year /size /retina /touchbar is this

or

what exact macOS you are currently running?

To trouble shoot further you can:

Try a SafeBoot https://support.apple.com/en-us/HT201262

Takes noticeable longer to get to the login screen, does a 5-15 minute disk repair before it fully boots up, and certain system caches get cleared and rebuilt, including dynamic loader cache, etc.

This test will tell you if third party interference; extensions etc are not loaded in safe boot mode.

Test issue in another user (or guest user) account  https://support.apple.com/en-us/HT204443

This will tell you if it a universal issue or isolated to your user/admin account. 

alex7375 wrote:

Yes, I did not know the two finger swipe worked in this app.

Thanks,

Now I no longer have to use the scroll bar on the MBP.

Two finger swipe works much faster and better than the scroll bar.

Good computing alex7375 !