Where is maildrop-2.1.0.tar.bz2 file?

http://prdownloads.sourceforge.net/courier/maildrop-2.1.0.tar.bz2 is given in your HOWtoForge instructions.

http://prdownloads.sourceforge.net/courier/ gives a range of files, but not the one above.

Which one is used then?

 

Use the latest stable version (currently 2.4.3).