One thing I'm slightly concerned about is the growing importance of sender based authentication schemes (such as SPF and DomainKeys); subscriptions placed via our web servers and our mirrors are liable to break in the not too distant future as these technologies become more widely adopted.
It's worth thinking about an alternative (but still convenient) approach to this process. Ideally (from a paranoid mail administrators point of view) we'd have the users browser send the subscription mail via a mailto: link. This looks and feels less convenient for the user, and it might not even be possible to subscribe to multiple lists in one-shot.
If it is impossible, and we really want to keep the current subscription stuff (I suspect Rasmus will be strongly in favour of that :-), then we could punt and deliberately break authentication for the subscription addresses. This still poses problems for mail originating from mirrors that has to transit through an intermediate authentication-enabled MX.
I guess you can't win them all...
This isn't high priority, but it would be great if one of you guys could start thinking about ways of shunting the actual sending of mail back onto the user, I'd much appreciate it.
Every mailing list subscription request submitted via the web interface is HTTP POST-ed to
http://master.php.net/entry/subscribe.php, so all the subscription mails are originated from there, regardless of the mirror you submit your subscription request. This is also a single point of failure for this service, but makes it a lot easier to maintain in these kind of problems you mention.
Goba