If you don't have the 'testing' repo enabled then you might want to enable it, reload the repo package lists, and then search slapt-get/gslapt again. If you DO have testing enabled then compiling from source might be the best solution.
I just checked the 'extra' repo and found mailx, msmtp, and ssmtp. You should be able to grab these via gslapt/slapt-get. Check which sources you have enabled and make sure you have 'extra' enabled, then refresh your package lists, and search again.