[Mailman-Users] Comcast, Yahoo, Hotmail blocking us

Mark Sapiro msapiro at value.net
Sat Sep 1 20:09:44 CEST 2007

Michael Anderson wrote:

>Suddenly, these major ISP's were refusing our connections. We are not on
>their blacklists. This all started on one day.
>After a great deal of investigation by my sysadmin and my University IT guys
>we have come to the conclusion that it might have been caused by updating
>our mail gateway machine. We upgraded from an antiquated PIX to one that is
>lightening fast. Plus we upgraded our bandwidth at the same time.
>So, what is happening is we are delivering the mail so fast that these ISP's
>are rejecting mail because they have thresholds in place that block large
>quantities of mail coming in from the same server in a short time.

I suppose that's possible, but before going down that road, I would
make sure that the domain that the server identifies itself as in SMTP
HELO or EHLO is the same domain returned by an rDNS lookup of its IP

I.e. if I connect from IP and identify myself in HELO as
example.com, an rDNS lookup of should return example.com. If
this is not the case, or if the rDNS returns a 'generic' name such as
DSL-10-0-0-1.example.com, this is sufficient reason for some ISPs to
block you.

>So, the question I have is - is there any way to set mailman so that it
>doesn't send all its copies of a message to yahoo.com for instance all at
>once? Can you break it up? We have 200 yahoo users which is over their

There is no throttling per se in Mailman. You can probably slow things
down a bit by setting


in mm_cfg.py which will cause Mailman to deliver each recipient to the
outgoing MTA in a separate SMTP transaction instead of sending in one
transaction with up to 500 recipients. You can accomplish the same
thing by setting


but if you're sending to 1 recipient per transaction, you might as well
get the advantage of VERP like delivery for better bounce detection.

If you have default Mailman settings, it is possible that Mailman is
delivering all 200 Yahoo recipients to the MTA in a single
transaction. Then, the MTA may or may not attempt to deliver all 200
to Yahoo in a single transaction which Yahoo may not like. If so, the
above settings may help.

If you want to slow things more than that, you need to investigate how
to do it in the MTA.

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

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