[Mailman-Users] another question about invitation confirmationsubject line/VERP

Mark Sapiro mark at msapiro.net
Wed Nov 28 03:26:16 CET 2007

Christopher Adams wrote:
>One concern - Charles mentioned how 'expensive' it was to use VERP
>with Postfix. I am wondering what kind of hit on the responsiveness of
>my Mailman server will be. I have over 500 lists and 250,000
>subscribers. If VERP being used for Mailman causes more mail activity,
>could this result in slowness of list message delivery?

The short answer is yes.

But let's be clear on what we're talking about.

First, we are not talking about VERP_CONFIRMATIONS = Yes. All this does
is remove the confirm token from the Subject: and put it in the From:
address and replace the Subject: with a more user friendly one. It has
no impact on performance because these messages are sent one recipient
at a time anyway.

What we are talking about is Variable Envelope Return Paths. I.e., each
recipient (e.g. recip at user.example.com) of a message (sent from the
listname at list.example.com list) gets a message sent with an envelope

  listname-bounces+recip=user.example.com at list.example.com

Then when a bounce is returned to that address, it is delivered to
listname-bounces at list.example.com and Mailman knows that the user that
bounced is recip at user.example.com without having to parse the actual
bounce message and regardless of where the message might have been
forwarded/aliased in between.

If mailman does this VERPing (e.g. by setting VERP_DELIVERY_INTERVAL =
1), then Mailman is required to send to each recipient in a separate
SMTP transaction instead of a few transactions with a few hundred
recipients each, because each recipient has a unique envelope from.
List personalization also requires this because each recipient gets a
different message. That's why Mailman has a setting for
VERP_PERSONALIZED_DELIVERIES since if the deliveries are personalized
to begin with, Mailman's VERPing doesn't cost extra.

The alternative is to have the MTA do the VERPing. In this case
delivery from Mailman to the MTA proceeds just as in the non-VERP
case, but delivery from the MTA to the recipient domains might require
more work. I.e., in the non-verp case, Mailman gives a message with
500 recipient addresses to the MTA in one SMTP transaction. If 25 of
these are aol.com addresses, the MTA may deliver the message to AOL in
one transaction with 25 recipients, but if the MTA is VERPing, it is
required to deliver to AOL in 25 separate transactions. So the impact
is greater than for non-VERP, but significantly less than if Mailman
does the VERPing. The problem is that unless the MTA can be configured
to VERP every outgoing message (which you may not want to do even if
you can), Mailman has to be patched to request that the MTA does VERP
for 'this' message.

As a point of reference however, there are lots of lists on python.org
and some are quite busy (e.g. Python-list at python.org) and they are
personalized and it seems to work OK. Brad has posted on this in the

Mark Sapiro <mark at msapiro.net>        The highway is for gamblers,
San Francisco Bay Area, California    better use your sense - B. Dylan

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