Using classic Outlook for Windows in business environments
A change to the email and OneDrive storage has been rolling out since February 1.
One change is OneDrive storage is now being called Microsoft storage.
Changes to Microsoft 365 email features and storage - Microsoft Support
This change adds your Outlook.com attachments and inline images to the OneDrive storage.
Check your OneDrive storage and the amount of storage used by the Outlook.com attachments and inline images will show.
You can check Mocrosoft and email storage here: https://outlook.live.com/mail/0/options/general/storage
If the OneDrive storage is over the limit, your email account will not be able to send and receive emails.