Before Gmail came along, Hotmail was pretty much the standard when it came to free email services. However, in 2013, Microsoft officially retired the service and migrated users over to Outlook.com, but at the same time allowed them to keep their Hotmail usernames and email addresses.
At the same time, the company imposed a 15GB storage limit, which is par for the course for free email services. Unfortunately for some users, it seems that for whatever reason, Microsoft has decided to finally do something about users who might have gone over those limits, with reports claiming that some Hotmail and Outlook.com users are being partially locked out of their accounts as a result of that.
While Microsoft is well within their rights to do so, what users are taking issue with is that ever since they were migrated over to Outlook.com 8 years ago, Microsoft did not necessarily warn them that they were over the limit. Instead, users are being locked out of their accounts and are unable to send or receive emails until they free up more storage.
It is unclear why Microsoft did not send warnings earlier, or if they did, how come some users missed out on it. In the meantime, users have the option of either deleting old emails to free up space, or sign up for Microsoft 365 which in addition to providing users with access to Office, it will also come with additional storage that they can use.