Qmail and ezmlm have built-in advanced mail loop prevention. However, some auto-responders are misconfigured and create a new message to the list for every message received. Since this is a completely new message, ezmlm has no way of knowing that it is not a post. This is very rare, but may happen. The symptom is that a “John is on vacation” messages will arrive at the list, go out to subscribers, only to be followed by another “John is on vacation” message, and so on.
To stop this, look at the sender of the “vacation” message. Unsubscribe the address.
Even rarer is that there is a mail loop somewhere on the Internet, and that it sends of messages to your list. Unsubscribing doesn't help. Your only recourse is to block the sender of the message from posting. Easiest is to use the “deny”(*) function with the ezmlm-make(1) ‘-k’ switch, then add the sender address:
% ezmlm-sub DIR deny badname@badhost
or if configured, send mail as a remote administrator to ‘listname-deny-subscribe-badname=badhost@listhost’. You could also add a line:
|/bin/echo -- "$SENDER" |/usr/bin/grep -iv '^badname@badhost$' || exit 99
to DIR/editor of the list.