Acknowledging and Responding to an Alert

To respond to or acknowledge an alert:

Select Open on the alert message toast:

OR Select the Message Center icon next to your name in the upper right corner.

AND then select Alerts category...

And the alert itself:
Select the green check mark to acknowledge that you have read the message.

The alert sender can see whether you have seen or acknowledged the alert.
Select the gray arrow to respond to the alert sender as a new conversation.
Enter the message and select Send as you would responding to a regular message.