[Mailman-Users] Mailman + Exim + strange headers

[Stephen J. Turnbull]

Natu writes:

 > When I had the problem, in addition to signing outbound messages, I
 > also increased the level of spam control.  In particular I have a
 > small number of users that forward their mail to gmail, and gmail
 > was blocking us because of the spam that was getting forwarded.  I
 > suspect that it may only have effected the forwarded email from
 > those accounts.  I also made efforts to increase spam detected and
 > thus reduced the amount of forwarded spam.

It sometimes happens that there is a way past a host's inbound
filters.  This shouldn't happen, of course, so when running mailing
lists you want to filter on the way in.

But it might be an "amusing" test to hook up the spam checker to the
*outgoing* MTA -- if it catches anything on the way out, then you have
a backdoor into your MTA somewhere.  For example, I trusted my
secondary MXes to filter (in fact the evidence suggests their
filtering was better than mine), until one day the spamchecker at one
of them fell over, and they continued to forward mail addressed to us
-- *including* the spam.

I happened to be online when it happened, or shortly after, so only a
few hundred (!) got through, mostly to invalid addresses (thank
heavens for really stupid spammers! at least that time :-).  It turned
out that *for me* there was no degradation of service when I stopped
whitelisting that MX, and I ended up just checking *all* mail always
regardless of my trust in the MX.  But YMMV, be careful -- spam
checking is resource intensive!

Also, if you're an ESP and your users keep their addressbooks on
Windows machines, you probably should be filtering outbound anyway.
In that case it's probably a good idea to keep your lists on a
separate IP from your other users, which allows you to filter only on
the way in (much more efficient).

 > With Yahoo, I was also getting complaints that I was sending them spam
 > (i.e. rejected mail from their smtp server).  I registered with yahoo's
 > complaint feedback loop, where they claim they will forward to you (the
 > registered Mail administrator for the domain) any email they receive
 > from your domain that they think is spam or that their users have
 > flagged as spam.  After I both turned on dkim and registered, I never
 > got a single complaint from them.

Cool!  Nice to hear that actually works (we've heard complaints here
about Yahoo!s responsiveness in the past).

 > One other thing that might cause problems with the freemail providers is
 > as follows:  If mailman sends a large number of messages to a freemail
 > provider that are almost the same, i.e. the exact same email, but
 > addressed to many different recipient, then they might think your are
 > sending bulk unsolicited mail.  Having your mailing list mail test
 > positive in DCC is probably a good indication that this is happening.  
 > There may be an option in mailman to ensure that outbound messages from
 > a single post don't all look identical.  If so, you could try turning
 > that on.

The option is "personalization".  Many lists use it so the user gets a
direct URL to their personal config page in the footer.

For the big ESPs like Yahoo!, just registering with their feedback
loop or bulk mailer list often helps here, too.  It may be worth
experimenting with that, because turning on personalization may be a
serious resource drain if you have hundreds of subscribers at one
domain.

 > Also, if you are dealing with lists that send high volumes of email to
 > gmail, destination specific metering of your outbound mail, as well as
 > adjusting the number of recipients in a single smtp transmission may
 > help.  If you are running postfix, discussions about how to implement
 > this can be found in the postfix archives.

All good advice!  Thank you for an excellent summary!

Steve