Error 5.7.1 when sending in outlook.it (Client host rejected: Access denied)

Hi Lu Greetings! Thank you for posting on Microsoft Community. I will be happy to help. I am a user just like you and only trying to advice/ assist in the best possible way to resolve your issue. This error message indicates that the recipient's email server is rejecting your emails due to an "Access denied" error. Let's figure out why this might be happening and how to resolve it: Possible Causes: Recipient's Server Restrictions: The recipient's email server may have strict anti-spam rules or blacklists that are blocking you. Your IP/Domain on a Blacklist: Your Outlook.it IP address or domain may be on a blacklist used by the recipient's email provider. Incorrect MX Record: It's possible (though less likely) that an incorrect MX record setting for your Outlook.it domain could cause delivery issues. Authentication Issues: Missing or misconfigured SPF, DKIM, or DMARC records can cause deliverability problems. Troubleshooting Steps: Check Sender Reputation: Use a reputation checker like [https://mxtoolbox.com/ ] Look up your Outlook.it sending IP or domain. Search blacklists like Spamhaus, Barracuda, etc., with your IP/domain to see if you're listed. Follow delisting instructions if necessary. Disclaimer: This is a non-Microsoft website or app. The page appears to be providing accurate, safe information. Watch out for ads on the site that may advertise products frequently classified as a PUP (Potentially Unwanted Products). Thoroughly research any product advertised on the site before you decide to download and install it. Contact the Recipient: If possible, reach out to the recipient directly (not through the problematic email address). Ask them to check their server settings and see if your email address or domain is blacklisted. Ask them to whitelist your address if possible. Verify MX Records: Make sure the MX records for your Outlook.it domain are pointing to the correct Microsoft Outlook servers. Your IT administrator or domain registrar can help with this. Check Authentication Records (SPF, DKIM, DMARC): If you manage your own email server, ensure you have SPF, DKIM, and DMARC records in place. Your email provider can help you set these up. Additional Considerations: Mass Mailing: If you're sending a large volume of emails to that address, it might be interpreted as spam. Consider spreading the emails out over time or using a dedicated email marketing service. Hope this helps and answer your question. Let me know if you need further clarification or assistance. Give back to the Community. Help the next person who has this issue by indicating if this reply solved your problem. Click Yes or No below. Kind Regards -Adedayo