Right, a fairly short CGI script should do this for you. Let me throw together a quick one and see how it looks:
-- copy from the next line on --
#!/usr/local/bin/perl
# Variables defined here.
$mailprog = '/usr/lib/sendmail';
$host = 'yourreplyaddress@domain.com';
# Get the sender's name, message subject and e-mail address from the form.
$name = "$FORM{'name'}";
$subject = "$FORM{'subject'}";
$email = "$FORM{'email'}";
$ipaddy = "$ENV{'REMOTE_ADDR'}";
# Assign the body of text from the form to a variable.
$body = "$FORM{'body'}";
# Send the e-mail off.
open (MAIL, "|$mailprog $email"

|| die "Can't open $mailprog!\n";
print MAIL "From: $host\n";
print MAIL "Subject: $subject\n";
print MAIL "Reply-To: $host\n\n";
print MAIL "$body\n";
print MAIL " \n";
print MAIL "This message was from the following address: $ENV{'REMOTE_HOST'} ($ENV{'REMOTE_ADDR'}).\n";
close (MAIL);
-- end of script --
You might want to add on some code that gives indication that the e-mail was sent successfully. Do some light reading on CGI/Perl and edit things as needed, but you should only really need to edit the very first line and the variable $host. Of course, you'll need a small HTML form to draw all those variables from as well.

PM me for additional help.