What do the different colored blocks (yellow, red, blue ...etc.) assigned to each e-mail mean?

I apologize for such a basic question.  This is my first day with eM Client but what do the different colored blocks (yellow, red, blue …etc.)  assigned to each  e-mail mean?

The blocks are category indicators. You can find a list at Menu > Tools > Categories.

Hi Gary,

Thanks for your reply.  With regards to an e-mail which I have received; if I get an e-mail that has been tagged with yellow, blue and red category blocks how do I know what the categories mean from this sender.  Is it not the sender who is doing the tagging or am I missing something?  Bill

The categories are not assigned by the sender. Typically you assign them, either manually or by Rules. But also if you are using an email provider like Gmail, they can be assigned by the server.

To find out what the colour means, just hover the mouse over the coloured block. It will tell you what the category is.