I havent seen both of those errors together.
With you dont have permissions to send to user, that sometimes is the user(recipient) has passed his mailbox limit.
Relaying denied smtp550 is not uncommon. Is it only for one user or to one domain ? If it was for all mail to the abc.com domain they might be using a real time black list that has your servers nated ip listed, try using www.dnsstuff.com to check. Another possibility is abc.com is using reverse dns to lookup your mx record and thats not working. I’m assuming the mail is coming from an exchange client and your smtp connector is setup ok.
To be honest you can get a much better explanation by doing a google on “550″ and “relaying denied”.