i'm sure I didn't delete this folder, accidentally or not. i really take care before deleting anything.
here's the output of the command which sendmail :
and as said before, postfix is installed and here is exactly what is installed in /usr/sbin :
ls -al /usr/sbin/sendmail*
lrwxrwxrwx 1 root root 34 jun 4 12:48 /usr/sbin/sendmail -> /etc/alternatives/sendmail-command*
-rwxr-xr-x 1 root root 23776 mar 8 17:19 /usr/sbin/sendmail.postfix*