smtp; 550 5.4.1 recipient address rejected: access denied.

Take this with a grain of salt because this is what came up on AI:

he SMTP 550 5.4.1 error in Qualtrics means the recipient's email server is actively blocking your message . This is a "hard bounce" typically caused by an invalid address, a disabled mailbox, strict spam filters, or domain blocks. [1, 2, 3, 4]

Common reasons for this error include:

• Invalid or Inactive Email: The recipient address does not exist or has been disabled (often seen with Microsoft Exchange servers).
• Blacklisting: The recipient's server has blocked the Qualtrics sending IP or your custom sending domain.
• Authentication Failures: Missing or incorrectly configured SPF, DKIM, or DMARC records can cause receiving servers to flag your emails as suspicious and reject them .
• Qualtrics Internal Block: Qualtrics might have placed a bounce block on that specific email address . [1, 2, 3, 4, 5]

How to troubleshoot and resolve this issue:

1. Verify the Email Address: Double-check for typos and confirm the recipient still uses that email address.
2. Contact IT Support: If this is a broader issue affecting multiple recipients within the same organization, have your IT team verify your DNS and Qualtrics Email Distribution Error Messages to ensure your custom FROM domain is properly authenticated. [1]
3. Check for Qualtrics Blacklisting: If you are certain the email is valid and they still aren't receiving it, the address may be on the Qualtrics system blacklist . You will need to reach out to Qualtrics Support with the specific bounced email address to have it unblocked . [1]

Here a couple of links:

Microsoft help article -  https://support.microsoft.com/en-us/topic/ndr-error-code-550-5-4-1-recipient-address-rejected-access-denied-c0e98a8e-81db-49c2-9bf1-32a1734d3e77

Qualtrics guide from North Dakota State - https://kb.ndsu.edu/it/129400

Qualtrics Help Guide - https://www.qualtrics.com/support/survey-platform/distributions-module/email-distribution/email-distribution-error-messages/