If you have several email accounts they are shown in a coloured "icon" with 2 letters which seem to be taken from the mail server name.
For example if the server name is gmail.com the icon is shown as "GM". If you have 2 Gmail accounts they are both shown as "GM". The only customisation that you can do is to change the colour of the "icon".
It would be nice to either select the two letters displayed or use an account image.
Note that the 3 accounts "GM" are all gmail.com accounts but the only display difference I can set is the colour.