Upstream error

I’ve setup E-mail routing. This seems to work paritally.
My destination e-mail is a custom domain hosted on outlook (not office 365).

Routing is:
USERatEXAMPLEDOMAIN → USER at DOMAIN2 (hosted on outlook).

When I send a test e-mail from USER at DOMAIN 2 (hosted at outlook) to the e-mail routed address (USERatEXAMPLEDOMAIN), I receive an error:

Cloudflare gives this error: Remote server returned '521 5.3.0 Upstream error

When I check the routing Activity log:
Unknown error: permanent error (550): 5.7.509 Access denied, sending domain EXAMPLE does not pass DMARC verification and has a DMARC policy of reject.

The DMARC record of my domain:
v=DMARC1; p=reject; rua=; ruf=; sp=reject; aspf=s; rf=iodef;

My SPF record:
v=spf1 include:OUTLOOK include:CLOUDFLARE -all

I’m not using DKIM (not supported for custom domains).

I’ve tested an outbound mail via Mailtester, score is a 9.0.
So that looks good. I’m puzzled to why this fails.

Does anyone have a clue/hint?