Hello
Thank you for your question and reaching out.
Please adhere to the advice in this help post about going over your storage limit. https://support.office.com/en-us/article/storage-limits-in-outlook-com-7ac99134-69e5-4619-ac0b-2d313bba5e9e
Please go there, click on any email, hit CTRL+A (this will select all), and then click on delete all after moving emails to the deleted things folder. Doing it in private browsing is preferable.
After that, kindly log in here https://outlook.live.com/mail/options/general/storage/mailboxStorage to confirm that you haven't utilised more than 15GB of space.
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