Restore sent mail from a backup

A few weeks ago all my sent mail disappeared from my mail box folders. The folder is there, it work OK for new mail, but all mail as of a early July is gone. All other folders are intact, just sent mail. I have a relatively rececent clone of my system, and wonder if there is an easy way to copy and merge the contents of the sent mail folder in the clone with the active one on my computer.

I don't know for certain, but I suspect this occured when I changed my email sever to gmail. I'm still using MacMail as my email client, but using gmail as the server.

I have an intel core duo Mac mini running OS 10.6.8, Mail version 4.5 The backup is not a "Time Machine" backup.

Any help would be appreciated.

Steven K. Smith