|  6 11/03 11/05 A. User <[log in to unmask]>
|                Last error: SMTP; 553 <[log in to unmask]>...
|                            Access Denied,name ULKYVM.LOUISVILLE.EDU
unknown to
|                            DNS

Hard to tell, BUT, mail to [log in to unmask] is "resolved" thus:

dig BELLATLANTIC.NET mx
 Ques: 1, Ans: 4, Auth: 8, Addit: 11
; ANSWERS:
ELLATLANTIC.NET.       122065  IN      MX      15 mail4.BELLATLANTIC.NET.
ELLATLANTIC.NET.       122065  IN      MX      15 mail6.BELLATLANTIC.NET.
ELLATLANTIC.NET.       122065  IN      MX      5 pmdfsrv.BELLATLANTIC.NET.
ELLATLANTIC.NET.       122065  IN      MX      15 mail3.BELLATLANTIC.NET.
; AUTHORITY RECORDS:
ELLATLANTIC.NET.       161815  IN      NS      watcherdc.ops.balink.com.
ELLATLANTIC.NET.       161815  IN      NS      world1.BELLATLANTIC.NET.
ELLATLANTIC.NET.       161815  IN      NS      world2.BELLATLANTIC.NET.
ELLATLANTIC.NET.       161815  IN      NS      world3.BELLATLANTIC.NET.
ELLATLANTIC.NET.       161815  IN      NS      world4.BELLATLANTIC.NET.
ELLATLANTIC.NET.       161815  IN      NS      world5.BELLATLANTIC.NET.
ELLATLANTIC.NET.       161815  IN      NS      world6.BELLATLANTIC.NET.
ELLATLANTIC.NET.       161815  IN      NS      world7.BELLATLANTIC.NET.

i.e., when the client SMTP tries to figure out the IP address of the mail
exchanger, it will query up to eight different domain name servers.
Instead of the IP address returned, the DNS returned four different host
names.  The client SMTP will then try to resolve THAT host name to an IP
address.  ASS-U-ME-ing that these table are all in-synch and correct, it
will then deliver to the loweest numbered MX host that is currently
available.  That might not be the same host as the user's mail box, and
internal mechanisms at the MX host should then deliver it properly.