Increased bounces at Microsoft domains: What's going on?

Florent Destors, Deliverability Manager at Marigold, recently found and shared some very good and timely information related to an increase of mailbox full (over quota) bounces that folks are seeing when sending to Microsoft domains. With his permission (thank you!), I'm sharing his info here:

Florent writes:

In case you noticed a drop these last weeks on your delivery rates towards Microsoft domains and see an increase in mailbox full bounces received, you should be aware that Microsoft recently changed their cloud storage calculation method.

Starting February 1, 2023, cloud storage used across Microsoft 365 apps and services will now include Outlook.com attachments data and OneDrive data. This update may reduce how much cloud storage users have available to use with their OneDrive. If they reach their cloud storage quota, their ability to send and receive emails in Outlook.com will be disrupted.

According to Microsoft, the new quota bar should have been gradually rolled out starting Feb. 1. However I can see an increase in bounces from March 23, and in some cases, it seems to impact quite a bunch of users.

Asynchroneous bounces received look like this:

554 5.2.2 mailbox full; STOREDRV.Deliver.Exception:QuotaExceededException; Failed to process message due to a permanent exception with message [BeginDiagnosticData]Microsoft Unified Storage is full. QuotaExceededException: Microsoft Unified

Find more information in the relevant Microsoft 365 support page here.