The error message "550 5.1.1 User Unknown" typically indicates that the email server encountered an issue while trying to deliver an email. It means that the recipient's email address is not recognized or does not exist on the recipient's email server.
Here are a few possible reasons for receiving this error message:
Incorrect email address: The email address you entered may contain a typo or an error, causing the server to fail in recognizing it as a valid address.
Email account no longer exists: The recipient's email account may have been deleted or deactivated, resulting in the "user unknown" error.
Domain name issue: The domain name part of the email address (after the '@' symbol) may not be configured correctly, or the domain itself might not exist.
Temporary server issue: It's also possible that the recipient's email server is experiencing temporary technical difficulties, and you may want to retry sending the email after some time.
To resolve this issue, you can try the following steps:
Double-check the email address: Make sure you've entered the recipient's email address correctly, without any typos or missing characters.
Verify recipient's account status: If you have communicated with the recipient before, confirm whether their email account is still active. You can try reaching out to them through an alternative method (e.g., phone call) to verify their email address.
Contact recipient or their IT support: If you are certain that the email address is correct, but you continue to receive the "user unknown" error, it's possible that there is an issue on the recipient's email server. In such cases, contacting the recipient or their IT support can help resolve the problem.