Steps:

Check Quota:

In the Microsoft 365 Admin Center, go to Exchange Admin Center (EAC).

Under Recipients, select Mailboxes and click on the affected user.

In the Mailbox Usage section, check if the mailbox has exceeded its storage limit.

Increase Mailbox Quota (Admin):

If necessary, increase the quota by going to the Exchange Admin Center.

Select Mailboxes, choose the affected user, and modify the storage limits.

Free Up Space:

The user can delete unnecessary emails or move them to an archive mailbox.

Alternatively, enable auto-archive to help manage mailbox size over time.