Messaging Gateway

    Attempted Delivery to: default-non-local-route  451 scanning error, please try later.      
    Attempted Delivery to: default-non-local-route  451 scanning error, please try later.      
    Attempted Delivery to: default-non-local-route  451 scanning error, please try later.      
    Attempted Delivery to: default-non-local-route  421 spamex02.webvis.net local verification problem - closing connection. 

Email is delivered only after 4hrs of delay with above error message. Any reason?

The '451 scanning error' is coming from the mail server we are trying to send the email to. You should investigate that server.

Mail Server of Intended recipient?

Pardon me, I only saw this response when checking for other SMG issue

Bruce,

Yes, server of recipient is giving you the 451 and 421 errors. Smtp 4xx errors are temporary errors - so smg has to queue and retry the delivery.

Thomas