Microsoft and cyber-security experts have ruled out a cyber-attack being the cause of the outage, which has affected Outlook, Teams and Copilot.
The tech firm said a large number of its servers had become “unexpectedly available”, meaning traffic was sent to the servers that remained online, putting them under strain.
“Just like on a motorway, when you block one lane, you have to put all of the excess traffic through just, say, one or two lanes,” Jake Moore, a cyber-security adviser at ESET, explained.
“Everything goes and backs up. It slows down, and that’s what we’re seeing here.”
He told Radio 4’s Today programme that this incident should “hammer home” the need for contingency communication networks that are “independent of others”.
“Unfortunately, we just put so much emphasis on just a few providers – so when one big one like this goes down, the knock-on effects are just huge.”
His sentiment was echoed by Nicky Choo, who said his cyber-security firm Mimecast used Microsoft tools but had built-in alternatives to prevent disruption.
Moore added that “good old-fashioned phone lines” seemed to be “the strongest form of communication, especially today”.
Some people were initially glad to hear about the outage – especially as it has occurred just before the weekend.
Martin told the that waking up to no new emails was “absolute bliss” until he realised “today is a crucial day to conclude a business deal”.