550 5.7.703 means your own Microsoft 365 organization stopped the message on the way out, because a recipient's address or domain matches a block entry in its Tenant Allow/Block List. Resend without the blocked recipient, and ask your email admin about the entry if it should not be there. No server outside your company ever saw the message.
How to read the result
Microsoft's text reads close to "messages to ... are blocked by your organization using Tenant Allow Block List", with the blocked addresses in the gap. Those are the recipients your organization blocks.
One match stops the whole message. Everyone else on the To or Cc line, inside or outside the company, missed it only because they shared it with a blocked address. A domain entry blocks every address at that domain, so a colleague's message to someone else there bounces the same way.
Microsoft's guide to Tenant Allow/Block List entries explains why: an entry added to keep a sender's mail out also stops users in the organization from sending to it.
What to do next
Resend to everyone else. Send the message again without the blocked recipients, so the rest get it today.
Ask why the block is there. Forward the bounce to your email or security admin. The Organization Management and Security Administrator role groups manage these entries in the Microsoft Defender portal.
Leave it if it is there on purpose. A domain blocked after a phishing attempt is blocked for a reason, so do not send from another account to get around your own security team.
For outreach, one person per message is the habit this code rewards: a blocked address then costs you one email, not the whole Cc line.