This would be the function of the Renewal= keyword.  You can define how
often everyone's subscriptions come up for renewal (monthly, bimonthly,
yearly, etc.)  You can also define how long they have to respond to the
renewal notification (common seems to be 14 days) and if they do not
reply, they are removed.
 
This is an excellent way to both weed out long-gone nomails as well as
giving those who can't remember how to unsubscribe (or forgot they were
subscribed and had gone nomail) a way to get off without having to ask
you or the general list "How do I get off of this list?"
 
I use a two-monthly setting on my lists (only six times a year) and it
works very well for keeping the subscription rolls current.  Actually,
the first time I ever enabled it, it turned out that about 35% of my
subscribers were long-gone nomailers.
 
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