Hello,
I get the message above when I try to send some mail. What is a default client server?How do I install it?
my guess is that you're clicking a mail-to link and your computer doesn't have a default mail client? Most people use web-based email nowadays so there really isn't really a need for an outlook or thunderbird client.
If this is the case either set up outlook/thunderbird/other mail app or use gmail, etc