Merle, SmarterMail has XMPP capabilities for chatting.  Use the following KB article to configure the Chat feature in SmarterMail.  From there your users can use the web interface for chatting or a 3rd party app.

Configuring chat feature
https://portal.smartertools.com/kb/a2698/enable-smartermails-chat-feature.aspx 

https://portal.smartertools.com/kb/search.aspx?search=xmpp 

Yes, I know it XMPP...Perhaps I should have stated my question better:
Since I don't believe SM17 (as well as 15.x) have anything that works for chat in a mobile environment, what  mobile apps are used by the folks on this board (if any)?

We used to use Trillian, it worked great, but SM doesnt seem to have the ability to chat with x number of random people, Trillian did work perfectly fine though across our iOS devices and desktops. However, we've since moved on to Discord due to the lack of multi-user convos.

This may have changed with SM 17, but I havent tried.