Hello Alyse Hart,
When you have messages going to quarantined, first thing to check is the reason it is getting blocked and sent to quarantine. It could be due to your Anti-spam policies, Anti-phish, Safe links policy or safe attachment policy.
You may also allow specific senders by following this guidance https://learn.microsoft.com/en-us/defender-office-365/tenant-allow-block-list-email-spoof-configure.
Additionally, If you want to try 3rd Party Phishing simulation follow this guidance https://learn.microsoft.com/en-gb/defender-office-365/advanced-delivery-policy-configure?view=o365-worldwide