It's possible for people to have the same display name, while having different email addresses. This becomes more likely in larger companies.

For example:

• John Doe (john.doe@company.domain)
• John Doe (john.doe2@company.domain)

@-mentioning @John Doe has unpredictable results. This is because display name is used to match the name:

        user = db_data['full_names'].get(name.lower(), None)
        if user is None:
            user = db_data['short_names'].get(name.lower(), None)

Since display name is not a unique attribute it would be much better to use the email address to identify the user as part of the send_message UI & API.