Two examples of error message below...
nlpi091.prodigy.net #553 5.3.0 nlpi091 - m9MISBuE005317, DNSBL:521< 18.104.22.168 >_is_blocked.__For_informa
mhb-03-ord.mailhop.org #550 Relaying denied ##
We are getting the above error when sending email to a few email addresses. Any idea as to why this is happening. We just upgraded to Excahnge 2007 and did not have this problem on Exchange 2003.