I have an Office365 email account that is being forwarded. In Settings>Mail>Forwarding the "Keep a copy ..." box is unchecked but a copy of the mail remains in my inbox. The result is a daily message from the server telling me my mailbox is nearly full. Why are emails continuing to be saved and will my Office mailbox eventually bounce arriving email?
Select the check box to Keep a copy of forwarded messages if you want to keep copies of your incoming email in your Outlook inbox. If you don't check this, once the email message is forwarded, it's deleted from your Outlook account.
Based off of this information and the information you provided, if the box is not checked then once the email is forwarded then the message is deleted from your Outlook account. Is there a chance that your email is being CC'd or BCC'd in the emails? If none of these are the case, then I would advise to reach out to Support for an account review.
Again I apologize for the delayed response and I hope you found this helpful. I hope you have a great day!