Hi @Ralf
Thank you for posting your question in Microsoft Q&A.
We understand that your business account cannot be added to Apple Mail.
Based on my research, this issue may be related to a known issue that Microsoft has acknowledged regarding Microsoft 365 email accounts disconnecting from Apple Mail. You can find more details here: Reconnecting your Microsoft 365 account to Apple Mail - Microsoft Support
You could try to follow these steps to troubleshoot it:
Re-enter Your Login Credentials
When prompted, click Edit Settings in the Apple Mail app and re-enter your login credentials.
If you don’t see this message in Apple Mail:
Open the Settings app.
Go to Apps -> Mail -> Mail Accounts.
Re-enter your login credentials.
Update Your Microsoft 365 Account in Apple Mail
1.Delete Your Microsoft 365 Account
Open the Settings app.
Tap Apps -> Mail.
Tap Mail Accounts -> Exchange.
Scroll to the bottom and click Delete Account.
2.Add Your Microsoft 365 Account Again
Open the Settings app.
Tap Apps -> Mail.
Tap Mail Accounts -> Add Account.
Select Microsoft Exchange and enter your credentials.
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