I could be wrong but there does not seem to be a built-in functionality
that
would support this activity. What you are talking about is "data mining"
or
"data extraction" because each message in Outlook represents its own
"record"
in the database that is your "Mailbox - ".
You interpret the original question differently than I, but there are tools
available that can scan Outlook folders and extract mail addresses from
them. The OP said, however than s/he wished to send messages to everyone
they've previously mailed, not everyone from whom a message was received and
clearly that's not possible by scanning messages in Outlook folders, since
there's no guarantee that the set of senders available there are people to
whom messages have been sent. I think the autocompletion cache is much more
accurate (up to 1,000 or 2,000 addresses, of course).