Depends on how you are using Sametime.
There are two ways:
Sametime Client : If you are using the Sametime stand along client then when you save the chat history it’s saved to a text file which can be mailed. You have to do it manually or set up Sametime to save all chats in a folder on your PC.
Sametime inside of Lotus Notes: If you are using the built in Client in Lotus Notes, and in version 8 it’s so much nicer. Then you can have Lotus Notes save the chats as email and then of course you can email them like they are email.