HTML apparently lets u put spaces in field names. i'm not sure its legal HTML, tho.
when PHP gets these, it replaces the spaces with _, so that the field names can be used within the php code as variable names.
then formmail just lists these fields names with thei values in the email u get.
the solution is to create a template, which also has the benefit of letting u format the email just the way u want.
here's the how to guide: http://www.tectite.com/fmhowto/plaintmplt.php
u can get fancier with HTML templates too.