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MAILDIRQUOTA(7) Double Precision, Inc. MAILDIRQUOTA(7)
NAME
maildirquota - voluntary Maildir quotas
SYNOPSIS
maildirmake {-q} {10000000S} {./Maildir}
DESCRIPTION
maildrop has a manual quota enforcement implementation that does use the quota facilities
of the host operating system (filesystem-based quotas). This is an option that is set at
configuration time, and may be disabled.
Using filesystem quotas sometimes leads to problems. After a hard quota is hit, most
commands issued by mail clients fail, including some commands that poorly-written mail
software may not expect to fail, and therefore be unable to gracefully handle the
unexpected failure.
With manual quotas, the only operations that fail are the ones that most mail clients
expect to fail, when the mailbox is full. However, filesystem-based quotas cannot be used
in some situations, such as virtual mailboxes. If this experimental feature is enabled,
approximate quota enforcement can be implemented by maildrop or deliverquota to whatever
extent it's possible to do so. Quotas are enabled by the -q option to maildirmake. Both
maildrop and deliverquota will observe any maildirmake-requested quota.
LIMITATIONS
This quota mechanism will only work as long as maildrop (or deliverquota) are the only
applications that deliver messages to the maildir, or as long as other applications
implement the same quota enforcement mechanism. The quota enforcement mechanism is
described separately in the README.maildirquota.html file in the source code distribution.
Quota enforcement will still work, to some extent, if there are other applications that
deliver or modify messages in the maildir. However, quota enforcement may not kick in
immediately when the maildir goes over quota, in fact the maildir can go over quota by a
noticeable amount. But eventually, as long as maildrop or deliverquota are responsible for
delivering the majority of messages, quota enforcement will kick in. Also, other sources
of messages may also result in noticeable performance degradation, because quote
recalculation will become noticeably more expensive.
Also keep in mind that this quota mechanism is generally useless if mail recipients have
direct access to their maildirs.
Finally even under the best conditions this quota enforcement does have a small chance of
a race condition where a maildir will go over quota by some amount. maildirs are designed
for speed, and this quota implementation was designed to have as little additional
overhead as possible, compared to regular maildirs. To enforce an exact quota you would
have to use some kind of a locking facility, which will impose a drastic performance
degradation on the overall maildir performance. A decision has been made to avoid locking,
with the only negative side effect being a possibility of going over quota in very low
probability situations.
A way to minimize the risk of going over quota is to set reasonable quotas, in relation to
maximum message sizes supported by your mail server. If you set your maildir quota to five
gigabytes, for example, it makes very little sense to configure your mail server to accept
messages up to three gigabytes in size. Your maildir quota should be chosen so that it
makes sense when compared to the maximum message size allowed by your mail server.
FOLDERS
This quota implementation is compatible with a popular extension to maildir which defines
individual mail folders within a single maildir. However, in order to create a compatible
folder, you MUST use the maildirmake command that comes with maildrop, and you MUST use
the -f option to maildirmake, giving the name of the folder. Do not use maildirmake and
specify the directory name of the maildir folder. Quota enforcement will not work if you
do that.
SEE ALSO
maildrop(1)[1], maildirmake(1)[2], maildropfilter(5)[3], deliverquota(8)[4].
AUTHOR
Sam Varshavchik
Author
NOTES
1. maildrop(1)
http://www.courier-mta.org/maildrop.html
2. maildirmake(1)
http://www.courier-mta.org/maildirmake.html
3. maildropfilter(5)
http://www.courier-mta.org/maildropfilter.html
4. deliverquota(8)
http://www.courier-mta.org/deliverquota.html
Courier Mail Server 07/24/2017 MAILDIRQUOTA(7)
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