I need to use web-based online chat in my website has the ability to keep the chat history for the visitor

I used some web-based chat services with monthly payment, all of them do not save the chat history for the visitors when they coming back again to my website.

So, I need to use web-based online chat in my website has the ability to keep the chat history for the visitor, and I know this feature needs an unique Identifier from the visitors for recognizing them when they come back again in another time.

I see most of the chats ask about the email before the visitor begin the chat, is there is any chat provider identify the visitor by their email when they come another time to the chat?