Thank you for contacting Yahoo.
I'm sorry if this has caused you any inconvenience and I would like to assist you with this matter.
Without a specific SMTP error message, we can't effectively troubleshoot the issue that resulted in the delivery failure.
Provide the following information in your response:
Detailed description of the problem (including when it started).
The sending IP(s) affected.
What types of emails are being sent from the affected IP(s)?
The output of a manual SMTP test with telnet from your mail server to our servers showing the SMTP conversation leading up to, including the rejection message. Telnet to this host and port number: telnet mta5.am0.yahoodns.net 25
Log messages from your mail server showing which IP you connected to and what responses you got from the remote server at the time you received the failures.
Without the requested information, we'll be unable to investigate the problem.
I am looking forward to your reply.
Regards,
Gaurav
Yahoo Customer Care