SET listname ACK FOR userid@host
should do it for a subscriber.
Adding to the list definitions DEFAULT-OPTIONS= will not change existing subscribers' options but should effect new and non subscribers.
If a subscriber who has this option is not receiving the ACK, then i suspect a problem with their mail system or config.
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Pete
To unsubscribe from the LSTOWN-L list, click the following link:
http://peach.ease.lsoft.com/scripts/wa-PEACH.exe?SUBED1=LSTOWN-L&A=1