Defer Sending Email using Mail.app

Try this AppleScript (using 10.7.3).

Select the messages you want to reply to before executing the script.

tell Application "Mail"
activate
set selectedMessages to selection
set countMessages to (count of selectedMessages)
if countMessages is equal to 0 then
    display alert "No Messages Selected" message "You must select a message first."
else
set theMessage to make new outgoing message with properties {visible:true, subject:"My Subject", content:"My Body"}
repeat with i from 1 to countMessages
    set theInput to item i of selectedMessages
set theName to sender of theInput
tell theMessage
    make new to recipient at end of to recipients with properties {name:theName}
end tell
end repeat
end if
end tell

I had a similar problem a few years ago and decided to move to a better email service. I chose Gmail, mainly because of the >10GB storage.

You can migrate your locally stored messages via IMAP. Once you set up your Gmail account, add the IMAP server to Mail. Then drag and drop all the messages you want to migrate on the Gmail inbox. IMAP will do the rest.

Adding the IMAP server to multiple devices will obviously keep them all in sync.

In general, I would recommend this over using a local option. IMAP is a superb protocol and I don't see the advantage of trying to sync the local archive in any other way.