From ITS:

As part of preparatory work for the Exchange Online Migration project ITS will be moving the Exchange Autodiscover service from the legacy Exchange 2010 servers to new Exchange 2016 servers. Some people may get the pop-up shown below, if so just check the “Don’t ask me about this website again” box and click Allow.

Although not mentioned in this message, my understanding is this will occur after 9.15 am Friday 21st June