Sprach- oder Videokommunikation in Echtzeit zwischen Benutzern auf mobilen Geräten oder Computern
This is happening in the Microsoft Teams app for personal use? The personal teams app doesn't have email.
If it is happening in Outlook:
It usually means the PST file was moved or is inaccessible. If you have a business account with a shared folder, the folder permissions changed.
What type of email account do you have configured in Outlook? POP, IMAP, Outlook.com, or Microsoft Exchange?
When you restart Outlook do you receive an error message?