I get an [ERROR CODE :550 - Verification failed] but only on 1 address...

When trying to send an email to a friend, I get an ERROR CODE :550 - Verification failed but only on this one address.

- Sending him an email with my private gmail address is successful.

- Sending myself an email with my SOHO address is successful.

- Sending other addresses an email with my SOHO address is successful.

His domain and MX is located behind Cloudflare. When I look at the records they are OK.

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info@his_email.nl, ERROR CODE :550 - Verification failed for <info@my_email.nl>
			Unrouteable address
			Sender verify failed

Reporting-MTA: dns; mx.zohomail.eu
Arrival-Date: Tue, 4 Mar 2025 11:28:41 +0100

Original-Recipient: rfc822; info@his_email.nl
Final-Recipient: rfc822; info@his_email.nl
Status: 550
Action: failed
Last-Attempt-Date: 4 Mar 2025 10:28:47 GMT
Diagnostic-Code: Verification failed for <info@my_email.nl>
			Unrouteable address
			Sender verify failed
Remote-MTA: dns; XXX.XXX.XXX.XXX