I get two copies of each mail.

Problem:

I get two copies of each mail.

Cause:

A bug in the cartitlib.cgi in CartIt 4.0 causes this error to come to light when using a Mail Transfer Agent (MTA) other than sendmail.

Solution:

Edit cartitlib.cgi

 
Find:
sub send_mail{
open (MAIL, "| $MAIL -t $to_mail") ||&system_busy;
 

Change the last line to read:
open (MAIL, "|$MAIL -t") ||&system_busy;

CartIt 5.x and up do not have this bug.