The Connected Account functionality, available on Exchange Online, allowing end-users to connect to external mail accounts (like Outlook.com or Gmail) to receive (and send) their emails using their Exchange Online mailbox is going to be deprecated.
Starting September 2018, all new mailbox on Exchange Online will no longer have the “Connected account” available, while the functionality will be fully removed on November 15th 2018.
So if you (or your end-users) were using this functionality, you will need to find alternative if you want to continue send and receive emails from your external mail accounts.