see https://support.microsoft.com/en-gb/topic/ndr-error-codes-550-5-7-703-550-5-7-705-550-5-7-708-and-550-5-7-750-f5675801-1846-4d1c-9599-5b4e26a9175d

This NDR is generated if you send an email message from an IP address that has a low reputation. The issue is more likely to affect new customers, such as those who have a Microsoft 365 trial subscription.

How do I fix this?

Only an email admin in your organization can fix this issue.

If you're an email admin, and you receive this error during a trial subscription, contact Microsoft Support to request an IP address exception until you assign Exchange Online licenses

.