Hi @Helen Slade ,
Welcome to our forum!
Do you receive the reminder on Outlook Web mail or in Outlook desktop client?
If it occurs on Outlook Web mail, please refer to this article to reduce your storage:
Mailbox storage limits in Outlook on the web
If the issue occurs in Outlook desktop client, please first check if the mailbox size is large:
If your mailbox is full, please refer this article to user Mailbox Cleanup to trim the size of your mailbox:
Hope the above help!
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