I understand Sam Varshavchik wrote.

But my target that is "How to extract user in a string=user1 user2 user3 ..." to
embed to loop.

There is difficult to extract each of user with pattern match result and
external script to embed.

Maildropfilter Document guide
(http://www.courier-mta.org/maildrop/maildropfilter.html):

to - deliver message to a mailbox

to expression

<-----snip----->

An expression that begins with an exclamation mark, "!" specifies a
whitespace-delimited list of E-mail addresses to forward the message to. The program specified by the SENDMAIL variable is run as
an external program, with the list of E-mail addresses provided as parameters to the program.

Now, solve this trouble with simple command in /etc/maildroprc:

to "!$string"

Maildrop is automatic to deliver message to all users that contains in string.

Regards,

--- Sam Varshavchik wrote:

The list of addresses in the To: or Cc: header has absolutely nothing to do with an individual message's message's recipients.

This mailing list message you are reading, was delivered to your mailbox yet your individual address appears nowhere in any of these headers.

Anytime anyone says they want to do something with the To: or Cc: headers, whatever they're trying to do it's always wrong. Whatever problem you are trying to solve, you have to find some other way to solve.