saving tagged messages - to what file?

Please,

when I am reading my mail using mutt, I can hit s to save the message
from a client and mutt assumes a file based on the FROM address.  That
is fine.

But when I save my reply it again assumes the from address - my own.
But this is not what I want.  I want to save the message, this time to
a file based on the TO: field.

I will tag a number of messages and I need to save them all to a file
based on my recipient's address, not my own.  There will only be one
recipient.  In other words I will end up with a file which is a total
record of all the mails from and to a particular client.

From the manual I suspect this can be achieved by Save Hook, but I
can't my head round it.

Thank you all for wonderful mutt it is an essential part of my little
CRM system.

Joe