Hi Rosemary,
Email providers, including Microsoft, use strict anti-spam and security filters to protect accounts.
Even though you only sent 12 addresses, using BCC and sending from a new account can trigger these filters because:
The account has no sending history or reputation yet.
Multiple recipients in one email look like spam patterns.
As a result, the system temporarily blocks the account to prevent abuse even if your intent is legitimate.
What You Can try is:
Check for Security Alerts
Sign in at account.microsoft.com and look for any verification prompts or alerts.
Unblock the Account
Go to Microsoft Account Recovery or the Outlook Unblock Page.
You may need to verify your identity via SMS or an alternate email.
Prevent Future Blocks
Avoid sending large recipient lists from a new account.
Send smaller batches or use Microsoft Groups or Mail Merge for events.
Add a clear subject and avoid suspicious-looking links.
If You Still Can’t Access
Contact Microsoft Support via support.microsoft.com and explain the situation they can manually review and unblock your account.
Let me know how it goes, Happy to help!