Lookup for "501 4.5.3"
Temporary Error - This status code indicates a transient issue and may resolve itself.
Basic Status Code Details (501)
Syntax error in parameters or arguments
Enhanced Status Code Details (4.5.3)
Too many recipients
More recipients were specified for the message than could have been delivered by the protocol. This error should normally result in the segmentation of the message into two, the remainder of the recipients to be delivered on a subsequent delivery attempt. It is included in this list in the event that such segmentation is not possible.