More often than not, a site that plans to run qmail is already running some other mail software on a Unix-ish server, and more often than not, that software is sendmail. This chapter walks through the steps involved in moving a mail system from sendmail to qmail.
Users tend to be upset when they can’t access their email, so it’s rarely possible to shut down the old mail system, spend a day getting the new system installed and tested, then turn the mail back on. Fortunately, you don’t have to. It’s easy to run sendmail and qmail in parallel on the same machine, delivering mail into the same mailboxes, until you’re satisfied qmail is working properly, and then shut sendmail down.
Any MTA receives mail through two routes: local and remote. On Unix systems, local mail is injected via the sendmail program, and remote mail is injected via SMTP. When you’re running qmail and sendmail in parallel, as long as /usr/lib/sendmail is a link to sendmail, local mail will go to sendmail, and as long as sendmail is listening on port 25, remote mail will also go to sendmail. While you’re testing, put qmail’s version of sendmail somewhere else, say /var/qmail/bin/sendmail, and run qmail’s SMTP daemon on port 26.
Once you’re happy with your qmail installation, move the original /usr/lib/sendmail to /usr/lib/sendmail.old (and similarly for any other links to it such as /usr/sbin/sendmail) and link the qmail version in its ...