I posted a question earlier, and am now supplying the answer
as given by the program author (for anyone else interested):
 
> Hi,
>  There's a new version (1.6) that compiles cleanly on
> X11R6... Its available from ftp.x.org in the
> /contrib/applications/xmail_1.6.tar.gz file.
>  If you don't have ftp access, please let me know
> and I'll mail the sources to you.
> 
> Jeff

moral: read the README... /jes