A common problem on mailing lists is users who have setup an "autoresponder", or software that automatically responds to all mail for them, that doesn't understand mailing lists. Autoresponders are most commonly used for "I'm on vacation"-style messages, but they might also be seen bouncing mail that appears to be spam or virus-infected. There are varying degrees of broken-ness, but there do not appear to be any limits on the stupidity of broken auto-responders.

The least harmful kind of stupid auto-responder is one that replies to the list administrator -- eg. if you are running foo-list@example.com, you keep getting "I'm on vacation" messages from one of your list members at foo-list- admin@example.com. The problems here might be 1) the user did not configure her autoresponder to exclude mailing lists, and 2) the autoresponder doesn't have a rate-limiting feature (eg. "Only send an automatic response to each sender once per day"). In this case, the autoresponder software is probably working correctly (in particular, it's sending responses to the envelope sender only), and the user has probably just configured it incorrectly. Since it's just you, the list administrator being bothered, how to deal with this is up to you. Disabling the user's subscription is an option (if they don't get any more list mail, they won't send you any more pointless "I'm on vacation" messages), but it's probably just as easy to ignore the messages (and send the user a polite note asking her to fix her autoresponder before she takes another vacation).

A more annoying kind of brain-damage is when every poster gets an inexplicable "I'm on vacation" message from someone he's never heard of (and who just happens to be on the same list). For example, let's say joe@some.domain sends mail to your list, foo-list@example.com. Mailman processes the message and sends a copy to everyone on the list, including bob@other.domain. If bob@other.domain has a stupid auto-responder that sends responses to some address in the header of the message (as opposed to the envelope), then joe@some.domain will get bob@other.domain's auto- response. joe will presumably be confused by this, since he doesn't know who the heck bob is. However, you -- the list admin -- won't find out about, unless you also post to the list -- or if joe figures out what's going on and tells you about it. In this case, the autoresponder software is broken, and sending a polite note to postmaster@other.domain, asking that the broken autoresponder be fixed or disabled, is legitimate. You should cc bob@other.domain so he knows what kind of trouble he's causing. If the problem persists, disabling bob's subscription is reasonable; you should send bob another note to let him know you have done so.

The worst kind of autoresponder brain-damage is when "I'm on vacation" messages get sent to the list itself. This should be inconceivable with Mailman (since the list address is usually only in the "To" or "Cc" headers), but remember: there is no limit to the stupidity of broken autoresponders. If this happens, the best option is to immediately disable the subscription of the user causing the trouble: you want to stop clogging your list with completely useless noise as quickly as possible. Again, you should send mail to the postmaster of the offending domain and the user responsible for the autoresponder, letting them know what kind of havoc their amazing broken autoresponder is causing.