Each Microsoft account comes with storage space, or mailbox, for both email and tasks. All Microsoft accounts start with 15 GB of storage space, with Office 365 Home and Office 365 Personal subscribers getting 50 GB of storage space.
If your mailbox is full, you won’t be able to sync your Microsoft To Do account. To make space in your mailbox so that your account can sync again, we recommend permanently deleting your deleted tasks.
Your tasks are stored on Exchange Online and are also visible in Outlook Tasks. To empty your deleted items folder in Outlook:
Sign in to Outlook’s desktop app with the same Microsoft aAccount that you use for To Do.
Navigate to your email folder list, then select Deleted Items > Empty Folder.
You can also create more space in your mailbox by deleting messages and attachments from your inbox. Learn more about deleting messages from your inbox.