Forum OpenACS Q&A: Re: References to qmail
Joel is right in his assesment. To answer his question, why does eCommerce use qmail? Only aD would know. My I'm guessing because they wanted a faster mailing solution. And probably because the rest of ACS also used qmail.
In OpenACS 4.x the eCommerce dependency on qmail is the exception to the rule and is slated to be removed. Moreover, qmail includes a replacement for sendmail which calls qmail-inject. As ns_sendmail calls sendmail, with the sendmail replacement in place the message will be send by qmail-inject after all.