Comment 4 for bug 605679

Okay, so this has been fixed in Debian's 1.10.1-3, but to work around it you can actually create a /bin/mail replacement that invokes "sendmail -t". I haven't run into the problem that Jozsef indicates -- but I can imagine it will break if www-user can't invoke the command for whatever reason.

The reason you should use sendmail -t instead of mailx is that the mail is sent with the headers as part of the main message body (i.e. it doesn't provide subject and recipient as commandline arguments).