qmail encourages a utilize of many innovations around mail (a few originated by Bernstein, others non), including maildir format mailboxes for storing messages (mbox files are too supported, & encouragement to migrate is given along by using the convienence to convert mbox mailboxes to maildir mailboxes) and a QMTP and QMQP protocols.

qmail's major contender come Exim and Postfix. Unlike qmail's competitiin, qmail has non been updated per creator for many years & users own instead are to rely on third person patches to trend lines recently functionality. Copyright status
qmail is Licence-Free Software, which itself [http://cr.yp.to/qmail/dist.html may be distributed] within source form or even in [http://cr.yp.to/qmail/var-qmail.html pre-compiled form (a "var-qmail package")]. Following, a bit of Linux distributions will non set up, & potentially not even include, qmail because by their system it classify such computer software when "non-free"^{One Two}. Since more MTAs come ordinarily involved inside distributions, by using their installation enforced by people distributions' pack management systems, this can own negatively affected qmail's popularity. Nevertheless, qmail users point out that these are "free enough" for anyone to utilise; a source code is publicly available & open for inspection & modification by users; & a licensing issues haven't stopped the prominent total of feature-enhancing augmentations or even many limited versions of qmail (that is to say [http://qmail.org./netqmail/ netqmail], [http://dqd.sourceforge.net/ dqd], & Debian's [http://packages.debian.org/stable/mail/qmail-src qmail-src package]) from existence published. Controversy
There exists a bit of contention among mail patterns operators above whether qmail is when standards-compliant when its creator claims. Critics allege a total of variations from either the SMTP standards, a bit of of which it claim produce qmail supplementary vulnerable to certain sort of abuse than more MTAs. [http://www-dt.e-technik.uni-dortmund.de/~ma/qmail-bugs.html] Others counter numbers of one claims by pointing out that a standards come ambiguous, & within a few events come at variance sustaining subsequent constituted better practice & so inordinate to exist as adopted by any mail software system.

For instance, critics comment in qmail's adoption of the different standard for bounce messages, QSBMF, to the of these around RFC 1894. Others counter by pointing out that RFC 1894 has just been adopted by a bit of mail systems, by owning more systems (even as qmail) using different bounce message standards; & by asserting that a condition of far flung forgery of envelope senders & a trend within recent years towards lone-hop shipping use actually undermined a foundations of RFC 1894 & rendered numbers of of its convolutions deliberate.

Another example of this argument is that of the behaviour of the SMTP Relay server in qmail while it comes to mail addressed to non-wanting mailboxes. Because of qmail's hard security partitioning between its SMTP Relay server and its local delivery professional (1 effect of this is that the spammer cannot enumerate user accounts by a dictionary attack, but this is non a resole cause for a hard security partitioning that diarrhea the SMTP Relay server as a user while forgoing any favorite privileges & without a means to affect more user files & processes.), & because its local delivery professional allows users & administrators to use "catch-all" wildcards & so extend a range of valid mailbox title at random, qmail's SMTP Relay server has no directly cognition of what local mailbox list come actually valid, & furthermore non necessarily plenty permissions to locate retired. When such, mail to non-wanting mailboxes (whose domain area come right, naturally) is accepted by qmail's SMTP Relay server, & qmail generates and sends bounce messages when a non-lacking mailbox title is late found, at the point of actual mailbox delivery.

Critics point out that qmail so sends far further bounce messages than some other MTAs, which around counterpoint give their SMTP Relay servers direct access to & noesis of local mailbox list & so allow the two to refuse mail addressed to non-lacking mailboxes; & that spam or worm mail messages often uses a system of sending messages to nwithin-missing mailboxes on intermediator systems placing a actual target mailbox in envelope sender addresses, relying upon a following bounce message from a intermediator to deliver the payload to the rattling target.

Others counter this criticism by pointing out that when yearn it trend lines the user-specifiable "address extension" mechanism sustaining wildcarding of a bit of form, possibly people more MTAs however own a equivalent condition of mail that just can not become found to exist as undeliverable until fallowing a SMTP Relay server has accepted it, and that so this just papers across a condition; that there is a fundamental conflict between preventing this kinda spam & a assure flexible project, that of these has there is no selection however to trade a a single for a more, and that the range of [http://www.faqts.com/knowledge_base/view.phtml/aid/29482/fid/206 six different patches available] for modifying qmail's SMTP Relay server & their incidental to results upon flexibility and security exemplify super easily a different trade-off; and that critics don't trend lines a abandonment of more advances in a state of a art in which the equivalent condition occurs (Even as running off the SMTP Relay server without privileges was an advance in the state of the art caused by security holes that allowed attackers to compromise local user accounts, a "Delivered-To:" is an advance around the state of the art caused per condition of mail loops amongst mailing lists consequent in explosions of mail. However spammers and worms can uses "Delivered-To:" headers to induce mail to become bounced at a point of actual delivery, & so to send their payloads to targets when bounce messages sent by intermediaries.).