Does Appian's mail function support the exchange of messages between users?Also, if it is not supported, is there a function that can exchange messages?
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No. If you want to exchange messages, you will have to use Tempo's news feature of build something on your own that supports this.
Well of course, there are various ways to build your own implementation for something that would do this. Depends in what "send messages" means - emails? A process could send emails. In-application? Just have "messages" written to DB, and some UI that tells users they have an internal message.
If only the site URL is disclosed to users, is it difficult for users (site users) to keep in touch via news?