When you’re chatting, you’ll see a toolbar button – Convert to Email. You can convert your chat into an email at any time by clicking this button. The new email will automatically be addressed to the contact you were chatting with…unless you don’t have their email address in your Contacts. In this case, you’ll be asked to complete the “To:” field manually.

In addition to Convert to Email, you’ll also see an option to convert a chat to email if/when the other person signs out. That way, nothing gets lost.