Does Time Machine keep backups of my emails?

It should, unless you exclude it from the backup.

I agree, Time Machine should keep backups of emails stored locally on a Mac! So if I open a Finder window of my back up drive (a WD My Book) I can navigate into the back up of a particular date i.e. /my iMac/Users/myName/Library... but I do not see Mail. It's a hidden folder on my iMac, which I can see holding down the option key... but that doesn't work when I navigate through my back-ups on the My Book. How do I see Mail in the back up drive?

I gave up on TM as a worthwhile backup solution, but dee if the gree demo of BackInTime can help...

http://www.tri-edre.com/english/backintime.html

Have you looked through Pondini's extensive TM help site?

http://Pondini.org/TM/FAQ.html