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Earlier this week, on Tuesday, July 18th, at approximately 4:47 AM EST, Microsoft communicated via @MSFT365status that they were looking into a service incident issue in which "some" users may have been unable to send Exchange Online emails. In this initial tweet, Microsoft was very quick with specifics as to the cause: a recent change to its free/busy infrastructure.
On Tuesday, April 25, 2023, at approximately 4:21 PM ET, Microsoft reported via tweet @MSFT365status that they were investigating an issue that was impacting Exchange Online connectivity for users in North America.
On March 1, 2023, at ~11:56 AM EST, Microsoft communicated via tweet (@MSFT365status) that they were investigating an issue in which users were unable to access Microsoft Exchange Online mailboxes.
On January 17, 2022, at ~02:06 am UTC, Microsoft communicated via tweet (@MSFT365status) that they were investigating an issue in which users were experiencing Exchange Online mailbox and calendar access issues.
For IT professionals and system admins with Microsoft admin center access, the incident # was EX315207.
On February 3, 2021 at ~10:45 pm UTC, Microsoft reported an issue that was causing some users in North America to experience issues with Exchange Online.
Many users quickly confirmed they that they weren't experiencing delays but rather emails altogether bouncing as well as error messages claiming service was unavailable.
It seems as if every summer something seemingly innocuous happens in a Microsoft datacenter halfway around the world and it spreads through the service like wildfire, taking down access for vast numbers of customers. It happened at the end of June last year in 2014, where Exchange Online and Lync Online were down for hours, and it has just happened again this week.