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  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?