Managing personal Outlook.com account settings, security, and privacy
A change to the email and OneDrive storage has been rolling out since February 1.
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.
If the OneDrive storage is over the limit, your email account will not be able to send and receive emails.
This has nothing to do with you using the stand-alone Outlook client to connect to Outlook.com.