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Bounce management

Automatic bounce management is a key feature in a modern mailing list server. Sympa provide automatic handling of delivery Status notification (DSN) and message disposition notification (MDN). This is mainly used in order to detect wrong subscriber email address and remove them from list. But you can use it to block list with too many wrong addresses or for message tracking.

The default Sympa configuration is good enough to automatically remove users that are in error for a long time and keep bounce rate low enough. This is important because list server with many invalid address may be blacklisted as a spam source. In addition, sending messages to wrong address is usefulness and may stress the relaying MTA.

Bounces are received at the mylist-owner address (note that the -owner suffix can be customized, see return_path_suffix), which should be sent to the bouncequeue program through aliases:

samplelist-owner: "|/home/sympa/bin/bouncequeue samplelist"

bouncequeue (see Binaries) stores bounces in a /home/sympa/spool/bounce/ spool.

Bounces are then processed by the bounced.pl daemon. This daemon analyses bounces to find out which email addresses are concerned and what kind of error was generated. If bouncing addresses match a subscriber's address, information is stored in the Sympa database (in subscriber_table). Moreover, the most recent bounce itself is archived in bounce_path/mylist/email (where bounce_path is defined in a wwsympa.conf parameter and email is the user email address). Note that the OTHER/ subdirectory gathers all bounces that could not get analyzed to determine the bouncing address.

When reviewing a list, bouncing addresses are tagged as bouncing. You may access further information such as dates of first and last bounces, number of bounces received for the address, the last bounce, etc.

With this information, the automatic bounce management is possible:

The automatic task eval_bouncer gives a score for each bouncing user. The score, between 0 to 100, allows the classification of bouncing users in two levels (level 1 or 2). According to the level, automatic actions are executed periodically by the process_bouncers task.

The score evaluation main parameters are:

  • Bounces count: the number of bouncing messages received by Sympa for the user.
  • Type rate: bounces are classified depending on the type of errors generated on the user side. If the error type is mailbox is full (i.e. a temporary 4.2.2 error type), the type rate will be 0.5, whereas permanent errors (5.x.x) have a type rate equal to 1.
  • Regularity rate: this rate tells whether bounces were received regularly, compared to list traffic. The list traffic is deduced from the msg_count file data.

The score formula is:

Score = bounce_count * type_rate * regularity_rate

To avoid making decisions (i.e. defining a score) without enough relevant data, the score is not evaluated if:

Bouncing list member entries expire after a given period of time. The default period is 10 days, but it can be customized if you write a new expire_bounce task (see expire_bounce_task).

You can define the limit between each level through the List configuration panel, in subsection Bounce settings (see bouncers_level1). The principle consists in associating a score interval with a level.

You can also define which action must be applied on each category of user (see bouncers_level1). Each time an action will be performed, a notification email will be sent to the person of your choice (see bouncers_level1).

VERP

VERP (Variable Envelop Return Path) is used to ease automatic recognition of subscribers email addresses when receiving a bounce. If VERP is enabled, the subscriber address is encoded in the return path itself, so that the Sympa bounce management process (bounced) will use the address the bounce was received for to retrieve the subscriber email. This is very useful because sometimes, non delivery report do not contain the initial subscriber email address but an alternative address where messages are forwarded. VERP is the only solution to detect automatically these subscriber errors. However, the cost of VERP is significant, indeed VERP requires to distribute a separate message for each subscriber and breaks the bulk emailer grouping optimization.

In order to benefit from VERP and keep the distribution process fast, Sympa enables VERP only for a share of the list members. If verp_rate (see verp_rate) is 10%, then after 10 messages distributed in the list all subscribers have received at least one message where VERP was enabled. Later, distribution message enables VERP also for all users where some bounces were collected and analyzed by the previous VERP mechanism.

If VERP is enabled, the format of the messages return path are as follows:

Return-Path: <bounce+user==a==userdomain==listname@listdomain>

Note that you need to set a mail alias for the generic bounce+* alias (see Robot aliases).

Message tracking

List owner can verify message delivery and disposition for each message using a special feature that can configured for a robot or a list.

This feature was added in version 6.2. It's a contribution from french army “DGA Maitrise de l'Information”. If the list is configured for tracking, outgoing messages include a “delivery status notification” request and optionally a “message disposition notification” request. This allows list owner to verify for each mail who did not received the message but also who received it and who displayed it.

Notification (both positive status and negative status are stored and can be used later to prove that a message was delivered or displayed by its recipient.

When active tracking override VERP parameter to 100%. Of course, tracking may be expensive.

Two different mode can be use for tracking :

  • DSN : Sympa will just require Delivery Status Notification . This is a ESMTP service that will not be viewed by recipients. This service may or may not be implemented by remote MTAs.
  • MDN : in that case Sympa will require both DSN and Message Disposition Notification. This service is usually prompted to recipient that may accept to send back a delivery or not. Because this may be surprising for subscribers and also because this may be used by spammers we suggest you not to use this feature unless subscribers are informed previously.

Tracking feature is accessible in archive web page when list owner display a message where tracking was activated. It require to support VERP so a plussed aliases is needed.

See list tracking configuration parameters

The following screen copy is a part of the tracking information of one message from a list archive. Email address are hidden in this image for privacy reason.

ARF

ARF (Abuse Reporting Format) is a standard for reporting abuse. It is implemented mainly in the AOL email user interface. AOL servers propose to mass mailer to received automatically the users complain by formated messages. Because many subscribers do not remember how to unsubscribe, they use ARF when provided by their user interface. It may be useful to configure the ARF management in Sympa. It is really simple: all what you have to do is to create a new alias for each virtual robot as the following:

abuse-feedback-report:       "| /home/sympa/bin/bouncequeue sympa@samplerobot"

Then register this address as your loop back email address with ISP (for example AOL). This way, messages to that email adress are processed by the bounced.pl daemon and automatically processed. If the bounced.pl service can remove a user, the message report feedback is forwarded to the list owner. Unrecognized messages are forwarded to the listmaster.

This feature is available as of Sympa 6.2.4.

manual/bounces.txt · Last modified: 2017/01/22 10:50 by dereckson@espace-win.org

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