Per RFC1893 (see http://ds.internic.net/rfc/rfc1893.txt ) a 5.1.2 error
is a permanent error (thus the 5.) which is defined as follows:

        X.1.2   Bad destination system address
           The destination system specified in the address does not
           exist or is incapable of accepting mail.  For Internet mail
           names, this means the address portion to the right of the
           "@" is invalid for mail.  This codes is only useful for
           permanent failures.

In other words, "no such host".

Nathan