For PVE VERSION 5.4 I'm using ssmtp and it worked for me by editing the file /usr/share/perl5/PVE/Tools.pm
Original line - open (MAIL, "|-", "sendmail", "-B", "8BITMIME", "-f", $mailfrom, @$mailto) ||
Corrected line - open (MAIL, "|-", "sendmail", "-B 8BITMIME", "-f", $mailfrom, @$mailto) ||
Regards