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

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.

  1. This message was created automatically by mail delivery software.
     A message that you sent could not be delivered to one or more of its recipients. This is a permanent error. 
    
    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
    
    

    • Sticky Posts

    • Pocket from Mozilla is closing shop. Don’t lose your favorites . Move them to Zoho Mail Bookmarks now! 📥🔖

      The end of Pocket shouldn't mean the end of your important links and content. Easily import them into Zoho Mail's Bookmarks and continue right where you left off. You can bring over your entire Saves, Collections, and tags just the way they are. Bookmarks
    • Zoho Mail POP & IMAP Server Details

      Hello all! We have been receiving a number of requests regarding the errors while configuring or using Zoho Mail account in POP/ IMAP clients. The server details vary based on your account type and the Datacenter in which your account is setup. Ensure
    • Workplace - Zoholics Europe Customer Awards

      Dear Zoho Mail Community! 📣 We're excited to announce that at this year's Zoholics events across Europe, we're holding customer awards for the first time ever! Workplace - Zoholics Europe Customer Awards This is your chance to shout about the amazing
    • Important update on Group Management | Zoho Mail

      Dear Zoho Mail Community, This post is to inform you all of the following updates on the Group Management in Zoho Mail. Group Member Addition: If a group member is added to an organization group in a Zoho Application, the member will be automatically
    • Mastering email etiquette and best practices to follow in Zoho Mail

      Hello Zoho Mail Community, We’re thrilled to invite you all to our upcoming webinar: Mastering email etiquette and best practices to follow in Zoho Mail. Join our live webinar for expert etiquette tips to enhance your professionalism and elevate your