Bounces

When a message to an email address bounces, Mailman’s bounce runner will register a bounce event. This registration is done through a utility.

>>> from zope.component import getUtility
>>> from zope.interface.verify import verifyObject
>>> from mailman.interfaces.bounce import IBounceProcessor
>>> processor = getUtility(IBounceProcessor)
>>> verifyObject(IBounceProcessor, processor)
True

Registration

When a bounce occurs, it’s always within the context of a specific mailing list.

>>> mlist = create_list('test@example.com')
>>> mlist.send_welcome_message = False

The bouncing email contains useful information that will be registered as well. In particular, the Message-ID is a key piece of data that needs to be recorded.

>>> msg = message_from_string("""\
... From: mail-daemon@example.org
... To: test-bounces@example.com
... Message-ID: <first>
...
... """)

There is a suite of bounce detectors that are used to heuristically extract the bouncing email addresses. Various techniques are employed including VERP, DSN, and magic. It is the bounce runner’s responsibility to extract the set of bouncing email addresses. These are passed one-by-one to the registration interface.

>>> event = processor.register(mlist, 'anne@example.com', msg)
>>> print(event.list_id)
test.example.com
>>> print(event.email)
anne@example.com
>>> print(event.message_id)
<first>

Bounce events have a timestamp.

>>> print(event.timestamp)
2005-08-01 07:49:23

Bounce events have a flag indicating whether they’ve been processed or not.

>>> event.processed
False

When a bounce is registered, you can indicate the bounce context.

>>> msg = message_from_string("""\
... From: mail-daemon@example.org
... To: test-bounces@example.com
... Message-ID: <second>
...
... """)

If no context is given, then a default one is used.

>>> event = processor.register(mlist, 'bart@example.com', msg)
>>> print(event.message_id)
<second>
>>> print(event.context)
BounceContext.normal

A probe bounce carries more weight than just a normal bounce.

>>> from mailman.interfaces.bounce import BounceContext
>>> event = processor.register(
...     mlist, 'bart@example.com', msg, BounceContext.probe)
>>> print(event.message_id)
<second>
>>> print(event.context)
BounceContext.probe

Processing

Bounce events are periodically processed via Bounce Runner to take actions for email addresses that bounce often. The first bounce in a day for an email address, in the context of a Mailinglist, increases the bounce score of their membership resource.

>>> from mailman.interfaces.usermanager import IUserManager
>>> user_manager = getUtility(IUserManager)
>>> bart = user_manager.create_address('bart@example.com')
>>> bart_member = mlist.subscribe(bart)

Initially, every member’s bounce_score is equal to 0.

>>> print(bart_member.bounce_score)
0

Once a normal bounce event is processed belonging to that member, the bounce score is increased by 1:

>>> event = processor.register(
...     mlist, 'bart@example.com', msg, BounceContext.normal)
>>> print(event.message_id)
<second>
>>> processor.process_event(event)
>>> print(event.processed)
True
>>> print(bart_member.bounce_score)
1
>>> print(bart_member.last_bounce_received)
2005-08-01 07:49:23

However, bounce_score is bumped only once for a day, any other bounces for the same day have no effect on the score:

>>> event = processor.register(
...     mlist, 'bart@example.com', msg, BounceContext.normal)
>>> print(event.message_id)
<second>
>>> processor.process_event(event)
>>> print(event.processed)
True
>>> print(bart_member.bounce_score)
1

Bounce score that is older than Mailinglist’s configured bounce_info_stale_after number of days older is considered stale. It is reset to 1.0 if a bounce event is received after that many number of days.

We pretend last bounce was received 10 days ago, more than MailingList’s bounce_info_stale_after days

>>> print(mlist.bounce_info_stale_after)
7 days, 0:00:00
>>> from mailman.utilities.datetime import now
>>> from datetime import timedelta
>>> bart_member.last_bounce_received = now() - timedelta(days=10)
>>> bart_member.bounce_score = 5

Now, another event after 10 days will reset the score:

>>> event = processor.register(
...     mlist, 'bart@example.com', msg, BounceContext.normal)
>>> processor.process_event(event)
>>> print(bart_member.bounce_score)
1

DeliveryStatus

If the bounce_score reaches the Mailinglist’s configured bounce_score_threshold, bouncing Member’s delivery is suspended:

>>> print(mlist.bounce_score_threshold)
5
>>> bart_member.last_bounce_received = now() - timedelta(days=1)
>>> bart_member.bounce_score = 4
>>> event = processor.register(
...     mlist, 'bart@example.com', msg, BounceContext.normal)
>>> processor.process_event(event)
>>> print(bart_member.bounce_score)
5
>>> print(bart_member.preferences.delivery_status)
DeliveryStatus.by_bounces

If Mailinglist is configured to do so, a notice is sent out the owners when a Member’s delivery is disabled:

>>> print(mlist.bounce_notify_owner_on_disable)
True
>>> from mailman.testing.helpers import get_queue_messages
>>> items = get_queue_messages('virgin', expected_count=1)
>>> print(items[0].msg['Subject'])
bart@example.com's subscription disabled on Test

VERP Probes

Instead of immediately suspending the delivery of a Member, Mailman can be configured to send VERP probes to the sender after their bounce score has reached the Mailinglist’s threshold.

>>> anne = user_manager.create_address('anne@example.com')
>>> anne_member = mlist.subscribe(anne)
>>> anne_member.bounce_score = 4
>>> anne_member.last_bounce_received = now() - timedelta(days=1)

Next bounce event for anne should trigger a probe which resets bounce_score:

>>> event = processor.register(
...    mlist, 'anne@example.com', msg, BounceContext.normal)
>>> from mailman.testing.helpers import configuration
>>> with configuration('mta', verp_probes='yes'):
...     processor.process_event(event)
>>> print(anne_member.bounce_score)
0
>>> print(anne_member.preferences.delivery_status)
None
>>> items = get_queue_messages('virgin', expected_count=1)
>>> msg = items[0].msg
>>> print(msg.as_string())
Subject: Test mailing list probe message
From: test-bounces+0000000000000000000000000000000000000001@example.com
To: anne@example.com
MIME-Version: 1.0
Content-Type: multipart/mixed; boundary="..."
Message-ID: ...
Date: ...
<BLANKLINE>
...
Content-Type: text/plain; charset="us-ascii"
MIME-Version: 1.0
Content-Transfer-Encoding: 7bit
<BLANKLINE>
This is a probe message.  You can ignore this message.
<BLANKLINE>
The test@example.com mailing list has received a number of bounces
from you, indicating that there may be a problem delivering messages
to anne@example.com.  A sample is attached below.  Please examine this
message to make sure there are no problems with your email address.
You may want to check with your mail administrator for more help.
<BLANKLINE>
You don't need to do anything to remain an enabled member of the
mailing list.
<BLANKLINE>
If you have any questions or problems, you can contact the mailing
list owner at
<BLANKLINE>
    test-owner@example.com
<BLANKLINE>
...
<BLANKLINE>

When such a probe bounces, their delivery is then suspended immediately:

>>> event = processor.register(
...     mlist, 'anne@example.com', msg, BounceContext.probe)
>>> processor.process_event(event)
>>> print(anne_member.preferences.delivery_status)
DeliveryStatus.by_bounces

Warnings and Unsubscription

When a Member’s delivery is disabled, they will received a configured number of warnings before they are removed as a subscriber of the mailing list.

>>> print(mlist.bounce_you_are_disabled_warnings)
3
>>> # The warnings are sent after a configured interval.
>>> print(mlist.bounce_you_are_disabled_warnings_interval)
7 days, 0:00:00

For now, anne hasn’t received any warnings:

>>> print(anne_member.total_warnings_sent)
0

Bounce Runner invokes BounceProcessor to sends these warnings periodically and removes members when max number of warnings are sent.

>>> processor.send_warnings_and_remove()
>>> print(anne_member.total_warnings_sent)
1
>>> print(anne_member.last_warning_sent)
2005-08-01 07:49:23
>>> print(bart_member.total_warnings_sent)
1
>>> items = get_queue_messages('virgin', expected_count=2)
>>> for item in sorted(items, key=lambda x: str(x.msg['to'])):
...     print('To: {}\nSubject: {}\n{}\n'.format(
...           item.msg['to'], item.msg['subject'], item.msg.get_payload()))
To: anne@example.com
Subject: Your subscription for Test mailing list has been disabled
Your subscription has been disabled on the test@example.com mailing list
because it has received a number of bounces indicating that there may
be a problem delivering messages to anne@example.com.  You may want to
check with your mail administrator for more help.
<BLANKLINE>
If you have any questions or problems, you can contact the mailing
list owner at
<BLANKLINE>
    test-owner@example.com
<BLANKLINE>
<BLANKLINE>
To: bart@example.com
Subject: Your subscription for Test mailing list has been disabled
Your subscription has been disabled on the test@example.com mailing list
because it has received a number of bounces indicating that there may
be a problem delivering messages to bart@example.com.  You may want to
check with your mail administrator for more help.
<BLANKLINE>
If you have any questions or problems, you can contact the mailing
list owner at
<BLANKLINE>
    test-owner@example.com
<BLANKLINE>
<BLANKLINE>

After Mailinglist’s configured bounce_you_are_disabled_warnings have been sent and another bounce_you_are_disabled_warnings_interval has elapsed:

>>> print(mlist.bounce_you_are_disabled_warnings)
3
>>> anne_member.total_warnings_sent = 3
>>> print(mlist.bounce_you_are_disabled_warnings_interval)
7 days, 0:00:00
>>> anne_member.last_warning_sent = (
...    now() - mlist.bounce_you_are_disabled_warnings_interval)

Now, the processor will unsubscribe anne:

>>> processor.send_warnings_and_remove()
>>> print(mlist.members.get_member('anne@example.com'))
None

If Mailinglist’s bounce_notify_owner_on_removal is True, owners will receive a notification about the removal. anne will also be notified about about the un-subscription, depending on how the list’s send_goodby_message is configured to True:

>>> print(mlist.bounce_notify_owner_on_removal)
True
>>> print(mlist.send_goodbye_message)
True
>>> items = get_queue_messages('virgin', expected_count=2)
>>> for item in sorted(items, key=lambda x: str(x.msg['to'])):
...     print(item.msg['to'], item.msg['subject'])
anne@example.com You have been unsubscribed from the Test mailing list
test-owner@example.com anne@example.com unsubscribed from Test mailing list due to bounces