On Tue, Apr 7, 2009 at 3:37 PM, Michuki Mwangi <michuki@swiftkenya.com> wrote:

Dear Skunkworks list members,

We wish to take this opportunity to extend our sincere apologies for the
outage experienced on the list in the past week.

This was caused by reasons beyond our control after the hosting server
was withdrawn from service for emergency maintenance and upgrades. Thus
the lack of advance notice on the outage.
o Over time we observed that most

[snip]

yahoo.com email addresses were
unsubscribed from the list due to delivery failures. We would wish to
monitor this going forward and fix the issue if it persists.

Hi Mich,

On another list that I manage, the problem with automatic unsubscription of yahoo\.* addresses also manifested itself. The reason appears to be the throttling algorithms applied by Yahoo! on their smtp servers to control junk mail.
This phenomena has been discussed on several forums and it's been found that Yahoo arbitrarily throttles incoming SMTP connections. If you send a "burst" of mail to yahoo servers, this throttling is applied and it sometimes even leads to their servers giving error code 550. This cumulatively adds to the score mailman applies to a member's failure threshold, which ultimately makes Mailman to autounsubscribe the member's address.

I use Exim as the SMTP server and somehow we (exim-users) agreed to send only a maximum of 5 messages per connection and this has saved us the agony from Yahoo!

In exim, we do http://lists.exim.org/lurker/message/20080818.185617.b5534450.en.html.
There must be a way of achieving the same using Postfix or any other sane MTA.

Just a HEADS UP.