I must be missing something in what you're saying.
If the server receives a message for myu...@mydomain.com and myuser's
mailbox is full... by default the server generates a bounce, I don't see any
way around that. It tries to send the bounce back to the sender from whence
it came, lets say that is
"enlarge_your_part-myuser=mydomain@theirdomain.com" and then their
server throws a 4xx and defers accepting the bounce, repeatedly, for a week,
until retries finally times out.If their server is deferring inbound
mail that doesn't sound like any sort of successful bounce management
strategy to me.
-Original Message-
From: Bill Cole via Postfix-users
Sent: Wednesday, August 23, 2023 9:17 AM
To: IUL Support via Postfix-users
Subject: [pfx] Re: Spam mails seen in logfiles question
On 2023-08-23 at 05:22:21 UTC-0400 (Wed, 23 Aug 2023 03:22:21 -0600) IUL
Support via Postfix-users is rumored to have said:
> Hi All,
>
> Have a legacy server that I've just taken over maintaining. It's set
> up with postfix that handles a small handful of email users. In
> looking through the logfile I'll frequently see emails bouncing (and
> the bounce messages being deferred so they just sit in the queue
> wasting retries).
You should fix that. It is a dangerous practice to accept mail and then
attempt to bounce it on a machine accepting mail from the Internet. This
generates a "backscatter" of bounces to forged addresses, often with
embedded spam and malware in the bounce messages.
> The email will be from
> some_spammy_text-myuser=mydomain@notmydomain.com and addressed
> to
> myu...@mydomain.com.
>
> The LHS always seems to have the same basic format ie. the underscores
> and the equal sign so it seems obvious that they're trying to
> accomplish
> something specific. Is it supposed to help them get past spam
> filtering,
> or get around some sort of bug?
It is called "variable envelope return path" or more often just VERP. It is
a common practice used in modern mailing list management that sends each
message with one recipient and a unique envelope sender to assure that any
delayed bounces (which are sent to the envelope sender, a.k.a.
'return path') can be positively matched to a user, so that the user can be
removed or suspended from a list if there are problems delivering to them
that can be understood from the bounce as likely to be repaired.
There are related variants on VERP that are used to encode an earlier return
path into a new one on forwarded messages (SRS) and as a trick to give every
message a unique return path and hence eliminate fake bounces
(BATV.)
> Can anyone enlighten me as to what they're trying to accomplish and if
> I should be doing anything configuration wise to block them from
> accomplishing it??
Don't block based solely on VERP use. VERP is almost universally used in
business-to-consumer bulk and transactional email, including messages which
are very much wanted and valuable. SRS and BATV, which look very similar,
are used to make other spam mitigation tactics possible.
> Is it supposed to help them get past spam filtering, or get around
> some sort of bug?
It's all about spam, like everything these days in email...
But it is NOT about getting around filters. It is about enabling rigorous
bounce handling (GOOD!) so that bulk mail senders can avoid sending de facto
spam and keep their lists clean. The similar-looking BATV and SRS are
tactics developed to allow other anti-spam techniques to work with fewer
errors.
--
Bill Cole
b...@scconsult.com or billc...@apache.org (AKA @grumpybozo and many
*@billmail.scconsult.com addresses) Not Currently Available For Hire
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