The servers don't seem to have a problem conducting a positive search
with my last name, but I don't get success with my first name.

Most of the servers just error-out when I use my fn. One server reported
"Error 500 - keyword too short"

But the same server has no problem with "tommi". Other servers find a
match right away.

Why the difference?

--- Thunderbird 2.0.0.24 (Windows/20100228)