Customize shown number of unread messages

Hi,

is there a way to customize the shown number of unread messages in the
task bar? Since I am subscripted to a lot of mailing lists and I receive
a lot of spam, I have virtually always unread emails. And since I don't
care whether these are 83 or 84, I would like to only see eg the sum of
unread emails in my INBOXes.

Cheers
Sebastian