It's on the server in their mailbox to do as they see fit. This is not something the admin will do.
-> Open their email client and if it's a great email client, press the resend button.
I've recently setup a Linux mail server here at home and it is working successfully. However, I have a user that normally leaves messages on the server (collects email via pop3d) and now wants me, the admin, to resend a particular message. I can see the message they are after in the spool file - /var/spool/mail/username - but do not have a clue as to how to send this one particular message other than copying it and pasting it into a new email that will come from of course. However, the user wants it to retain its format so that should they choose to respond, the author would not be any the wiser.
Is this doable?