If it's happening when the laptop is off then it isn't originating there. Check the Blackberry or the Yahoo account.
This often happens and although it appears that your (or your wife) is to blame it could be the result of an old bug that has floating around for years that infects machines and spoofs the senders email address when sending multiple emails in an effort to propagate itself.
This could be on any machine where your wife's email address is housed in the address book, so it could be anyone's machine, anywhere.
As a precaution you could run the FREE version of THIS software on the laptop. Update it before running and let it remove anything it finds and reboot if it asks.