Mail delivery failed: returning message to sender

[questfix]

Script URL: http://www.truthvisionary.com/guestbook/gbook.php
Version of script: 1.7 with some modifications made from reading the forum
Hosting company: 1and1.com
URL of phpinfo.php: http://www.truthvisionary.com/guestbook/phpinfo.php
URL of session_test.php: http://www.truthvisionary.com/guestbook ... n_test.php
What terms did you try when SEARCHING for a solution:

SMTP error from remote server after transfer of mail text:
host mx1.seanic.net[xxx.xx.xx.xxx]:
451 See http://pobox.com/~djb/docs/smtplf.html.

Write your message below:
When I replied as the admin to a user's comment, I received the above error message, three times. The user never received my reply. I've searched for this error on the net but do not completely understand what it means. I searched the forum but there is no post that precisely explains this error or where to look for the answer. I suspect it has to do with how the server processes mail and perhaps, some of the code in the script that might need changing.

[Klemen]

Do you get that error if you try this gbook.php file?
http://www.phpjunkyard.com/extras/gbook17_CRLF.zip

If this fixes the issue, you will either need to modify this file or change all "\n" chars used when generating email message and headers into "\r\n"