I recommend submitting a ticket on this Brian. Adding a Cloudflare IP should have no interaction with the from headers in your email client and we'd love to see the supporting details on this. Do you mean block list in the Outlook client, blacklist in SmarterMail, or firewall on your network?

Firewall on the network between outlook and the server.

Interesting! Does your firewall engage in SMTP filtering or any type of email scanning? I could see a scenario where the attempt to block/hide traffic to that IP could result in the headers becoming corrupted. If you turn off just those features in your firewall temporarily as a test it might help to confirm that.

If I unblock it, everything works fine.

But what traffic does my outlook depend on from a cloudflare ip?