With Postfix versions before 2.3, the
sendmail(1) command uses
the -V command-line option to request VERP-style delivery. In order
to request VERP style delivery with Postfix 2.3 and later, you must
specify -XV instead of -V.
The Postfix 2.3
sendmail(1) command will recognize if you try
to use -V for VERP-style delivery. It will do the right thing and
will remind you of the new syntax.