If you want to store the users' emails as well, you can add another field to the MongoDB document to store the email address. For example, you could modify the document format to look like this:

{
"src_username": "...",
"dst_username": "...",
"email": "...",
"date_time": "...",
"message": "..."
}

Then, when a user logs in or registers, you can prompt them to enter their email address and store it in the corresponding field in the database. You can also modify the UI to display the email address alongside the username in the chat history or in the "target_username_textbox".
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