This occurs when you use Stuffit Expander to extract the source code. Insted, make sure to extract the archive with either OpenUp or from the terminal with the following command:
% tar -zxvf GNUMail-X.Y.Z.tar.gz
2 Download
2.1 All files I downloaded are named download.cgi or mirror.cgi.
(MacOS X)
Some browsers have problems with our CGI scripts (or is it the opposite? ;). You have two solutions:
You use Netscape/Mozilla or Internet Explorer to download the releases or
You rename the file with the proper extension (either .tar.gz or .dmg)