This error means that Office 365 has added the user to the blocked senders' list because this person's emails have been recognized as spam.
The most common reason for this is that your email address is suspected of sending spam and it's no longer allowed to send messages outside of your ...
that sending account has been banned due to detected spam activity.
When you send an email and receive a delivery failure message, there could be ... your message has been intentionally blocked by a spam or blacklist filter on ...
Three days ago there were ~ 360 messages from "Microsoft Outlook"
To fix delivery has failed to these recipients or groups error make sure the email account is not compromised. Then check if the account is blocked for spam.
Generating server: DB5EUR01HT215.mail.protection.outlook.com
The issue should also go away after the GAL has been rebuilt and the address book
when Outlook delivers the message, and therefore the lookup could fail.
... valid sender. Delivery has failed to these recipients or groups.