I can think of the mail server actually refusing the mail due to authentication issue or connectivity issue or even IP filter issue.
Please check if you can send a test mail from within ePO and check mail server definition settings.
Thanx for that. I verified that the email server is relaying the mails . All is fine .
I restarted the Mcafee services , and hey presto it started sending the responses.
Surely restarting the services every time to "resolve" the issue is not kosher