Postfix: /usr/sbin/sendmail: No such file or directory - why?
Solution 1:
The postfix package on Debian actually includes an /usr/sbin/sendmail executable that functions as a compatibility interface (see man sendmail when postfix is installed).
Check ls -l /usr/sbin/sendmail* in case it was renamed or reinstall the postfix package so you get the wrapper in place.
Solution 2:
If you do not have /usr/sbin/sendmail, check if after installing postfix package, you have:
/usr/sbin/sendmail.postfix
If you do, simply make a symlink:
ln -s /usr/sbin/sendmail.postfix /usr/sbin/sendmail