List of recipients

I need to compile a list of users receiving messages matching a certain 
condition.  Instead of collecting the messages themselves and manually noting 
the recipients address, I would like to use procmail for the task.


I know how to extract a "To:" address (if there is one) with:

:0 h
to=|  formail -x "To:"

How can I append this value to a file?


Or, is there a better way to go about this?

Thanks,
Brian