2. Orientation

2.1 How many people use qmail?

qmail is used by hundreds of ISPs and thousands of other sites. Some high-profile users:

• ONElist, http://www.onelist.com handles millions of deliveries every day using qmail.
• The largest non-LSMTP LISTSERV host, listserv.acsu.buffalo.edu, has been running qmail since November 1996. It has handled two hundred million deliveries since then.
• Hotmail, http://www.hotmail.com with thirty million users, has been using qmail for outgoing mail since 1997. (Reportedly, after Microsoft purchased Hotmail, it tried to move Hotmail to Microsoft Exchange under Windows NT. Exchange crashed.)

There are more than 1000 people on the qmail mailing list.

2.2 What are the most common reasons for using qmail?

Here are the top ten reasons I've heard from qmail users.

1. Security. qmail doesn't let intruders into your machine.
2. Reliability. qmail never loses mail.
3. Speed. qmail delivers your mail much more quickly than sendmail, without compromising reliability.
4. Low memory use. qmail can handle dozens of simultaneous deliveries on an old 16MB 486.
5. User-controlled mailing lists. Users don't have to pester the system administrator to create new lists.
6. Virtual hosts made easy. qmail pioneered multiple-domain support.
7. Straightforward administration. qmail works with a minimum of fuss.
8. Flexible program deliveries. qmail provides a powerful interface for external mail processors.
9. Variable Envelope Return Paths. This feature (used by ezmlm, http://pobox.com/~djb/ezmlm.html) provides 100% automatic bounce handling for mailing lists of any size.
10. The maildir format. This feature makes it easy to set up high-volume distributed POP toasters (see http://pobox.com/~djb/qmail/toaster.html.)

2.3 Is qmail compatible with sendmail?

Yes. qmail supports

• .forward, ---see question How do I use sendmail's .forward files with qmail?.
• /etc/aliases ---question How do I use sendmail's /etc/aliases with qmail?.
• binmail deliveries to a central mail spool in the usual mbox format.
• /usr/{lib,sbin}/sendmail interface for mail injection.
• normal UNIX user database in /etc/passwd.

There is a checklist, http://pobox.com/~djb/qmail/sendmail.html for large sites moving from sendmail to qmail.

2.4 What operating systems does qmail support?

qmail works on practically all UNIX systems: AIX, BSD/OS, FreeBSD, HP/UX, Irix, Linux, NetBSD, OpenBSD, OSF/1, SunOS, Solaris, etc. It automatically adapts itself to new UNIX variants.

qmail does not support Windows NT.

2.5 Is there really a cash reward for security holes?

Yes. See http://pobox.com/~djb/qmail/guarantee.html.