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add a section on greylisting to pmgconfig.adoc

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While we mention greylisting as available feature in pmgintro.adoc, we
should also document a few more technical details of its workings
in PMG (e.g. how long a seen triple is kept before expiring).

Signed-off-by: Stoiko Ivanov <s.ivanov@proxmox.com>
Reviewed-By: Aaron Lauterer <a.lauterer@proxmox.com>
This commit is contained in:
Stoiko Ivanov 2020-04-23 12:23:28 +02:00 committed by Thomas Lamprecht
parent 6994b407f3
commit d41aa03929
1 changed files with 43 additions and 0 deletions
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pmgconfig.adoc
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@ -358,6 +358,49 @@ NOTE: Since before queue filtering is currently incompatible with the
editing '/etc/pmg/pmg.conf'.
[[pmgconfig_mailproxy_greylisting]]
Greylisting
~~~~~~~~~~~
Greylisting is a technique for preventing unwanted messages from reaching the
resource intensive stages of content analysis (virus detection and spam
detection): By initially replying with a temporary failure code ('450') to
each new email, the {pmg} tells the sending server that it should queue the
mail and retry delivery at a later moment. Since certain kinds of spam get
sent out by software, which has no provisioning for queueing, these mails are
dropped without reaching {pmg} or your mailbox.
The downside of greylisting is the delay introduced by the initial deferral of
the email, which usually amounts to less than 30 minutes.
In order to prevent unnecessary delays in delivery from known sources, emails
coming from a source for a recipient, which have passed greylisting in the
past are directly passed on: For each email the triple '<sender network,
sender email, recipient email>' is stored in a list, along with the time when
delivery was attempted. If an email fits an already existing triple, the
timestamp for that triple is updated and the email is accepted for further
processing.
As long as a sender and recipient do communicate frequently there is no delay
introduced by enabling greylisting. A triple is removed after a longer period
of time, when no mail fitting that triple has been seen. The timeouts in {pmg}
are:
* 2 days for the retry of the first delivery
* 36 days for known triples
Mails with an empty envelope-sender are always delayed.
Some email service providers send out emails for one domain from multiple
servers. To prevent delays due to an email coming in from 2 separate IPs of
the same provider the triples store a network ('cidr') instead of a single IP.
For certain large providers the default network size might be too small. You
can configure the netmask applied to an IP for the greylist lookup in
'/etc/pmg/pmg.conf' or in the GUI with the settings 'greylistmask' for IPv4
and 'greylistmask6' for IPv6 respectively.
[[pmgconfig_mailproxy_transports]]
Transports
~~~~~~~~~~