How can I tell if my contacts are available?
A visual representation 
of your contacts' availability, known as online presence or status, indicates if a person is online and available for a real-time conversation.
These indicators appear for anyone in your Outlook contacts who has an instant messaging address in the IM address text box.
Available
Busy
Do not disturb
Away
When you open a message in Outlook 2016 or 2013, or when it appears in the Reading Pane, the online status of a contact is shown:
next to the sender's name
next to each name on the To and Cc lines
in Contacts
Send an instant message
You can send an instant message to any person whose online status indicates anything except Offline.
Next to the person's name, click the online status indicator.
On the contact card, on the click the Instant Message or Send an IM icon, as represented by a speech bubble.
Type your message in the lower section of the window, and then press Enter to send.
If you have an email message open, you can reply with an instant message to the sender or the sender and all recipients of the message. On the Message tab, in the Respond group, click IM. On the drop-down list, click Reply with IM or Reply All with IM.
Turn online status on or off in Outlook
You control how your online status is displayed to other people. For example, if you are unavailable, you can change your online status to 
Busy.
Click the File tab to open the Backstage view, and then click Options.
On the People tab, under Online status and photographs, select or uncheck the Display online status next to name check box.
Click OK.
Services compatible with Outlook
Outlook can be used with several instant messaging services, such as Microsoft Skype and Lync. With these services, you can communicate with your contacts in real time as you would during an in-person conversation. Additional collaborative features are included with each service also.
For an Outlook contact, the online status is displayed when you rest the pointer on the person's email address. In a new meeting request, the online status is displayed when you rest the pointer on the attendee's name.
Online status is available for any person whose instant messaging email address you have added to your instant messaging contact list. In addition, online status is shown for people using Lync, regardless of whether or not they are on your instant messaging contact list.