This article describes:
Your message couldn't be delivered because you weren't recognized as a valid sender. If a user continuously sends emails that are classified as spam from Exchange Online, generally to external users (non-vt.edu), they can be added to the Restricted Users list which will restrict the user from sending email. However, the user will still be able to receive it. The user will be listed in the service as a bad outbound sender.
Microsoft has a limit on the number of emails a Microsoft 365 account can send per day (see Exchange Online limits for these limits). When a Microsoft 365 account starts to exceed this limit, Microsoft’s spam algorithms can consider the account to be a spammer. At this point, the account is put on the block list.
The account holder will receive a Non-Delivery Report (NDR) that states:
Delivery has failed to these recipients or groups:
Your message couldn't be delivered because you weren't recognized as a valid sender. The most common reason for this is that your email address is suspected of sending spam and it's no longer allowed to send email. Contact your email admin for assistance.
Diagnostic information for administrators:
Generating server: X.Y.prod.outlook.com [USER@DOMAIN.COM]
Remote Server returned '550 5.1.8 Access denied, bad outbound sender AS(42003)'
Virginia Tech has no control over these limits or the decision to block the account.
The resolution to this problem involves several aspects.