Sending mails using a zoho user name - noreply@mydomain generates error - sender domain invalid.

Sending mails using a zoho user name - noreply@mydomain generates error - sender domain invalid.

I'm sending mail from php -  mail($to, $subject, $message, $from ). But the mail bounced back with error:
Sep 16 22:52:20 web postfix/smtp[7680]: EB5984275B: host mx2.optonline.net[167.206.4.79] said: 453 4.5.4 Sender domain invalid.  Please try again. (in reply to MAIL FROM command)
Sep 16 22:52:20 web postfix/smtp[7680]: EB5984275B: to=< xxxxxx@optonline.net>, relay=mx1.optonline.net[167.206.4.77]:25, delay=21573, delays=21573/0.02/0.1/0.01, dsn=4.5.4, status=deferred (host mx1.optonline.net[167.206.4.77] said: 453 4.5.4 Sender domain invalid.

I've a valid registered domain. I'm sending mail from my AWS server, and Amazon folks reported that they removed all restriction. Don't understand why mails are bouncing back. Do I need configure anything further in Zoho? Can anyone please help. ...