Dear Melanie Peterson,
Based on your description, kindly try these steps:
Stop attempting to send email temporarily.
Change your password immediately, especially if you do not recognize the sent messages.
Review Outlook’s Sent Items, Deleted Items, forwarding settings, inbox rules, and recently installed apps for anything unfamiliar.
Sign out of unknown sessions and make sure multifactor authentication is enabled.
Next, please contact your IT Admin for these steps:
First investigate and secure the mailbox. Reset the password, enable MFA, review sign-in activity, inbox rules, forwarding, delegated permissions, and suspicious applications.
Open the Restricted entities page in Microsoft Defender.
Go to Email & collaboration > Review > Restricted entities.
Search for the affected mailbox.
Select it, choose Unblock, review the recommendations, select Next, then Submit > Yes. Appropriate Security Administrator or Exchange permissions are required.
Send a new test email to one recipient.
In case you do not know who your IT Admin is, kindly refer to this article: How do I find my Microsoft 365 admin? | Microsoft Support
I hope the information shared helps point you in the right direction.