So what are the little coloured (colored) rectangles in each message? (I'm new to this email client)

So what are the little coloured (colored) rectangles in each message?  (I’m new to this email client)

It is related to Categories   I do not use them so i ignore them   I remove them
one can change what is displayed  by right click  when mouse on top line (like subject)  and one can add or remove what is displayed    and can also apply the change to all folders

It sound like you are maybe a Gmail user?  If you are asking about the yellow markers, those are the “important” markers generated by Gmail itself and passed along to eM Client.

The coloured rectangles are categories assigned to that email. In the case of GMail, they are labels like important, inbox, sent etc.

Hovering over the rectangle will indicate what the category is.

OK Thanks.  Saved me some wondering