Since it isn't very obvious when superficially browsing the web pages just the remark that this is based on the GNUstep development framework according to https

Since it isn't very obvious when superficially browsing the web pages just the remark that this is based on the GNUstep development framework according to https://www.sogo.nu/support/faq/how-do-i-compile-sogo.html https://www.sogo.nu/support/faq/how-do-i-compile-sogo.html . This is propbably a testament for the maturity of GNUstep.