4.4.7 NDR Messages

I have a user that received mutiple 4.4.7 error messages sending calendar requests.  Mutiple users 3 diffrent domains.  Across the Country diffrent.  From what I understand 4.4.7 is usually a probome on the receiving end.  But this seems excessive.

Thanks!
_______________________________________________________
Reporting-MTA: dns;server.sender.com

Final-Recipient: rfc822;user1@domain1.com
Action: delayed
Status: 4.4.7
Will-Retry-Until: Sat, 18 Apr 2009 16:54:49 -0700
X-Display-Name: user1@domain1.com

Final-Recipient: rfc822;user2@domain1.com
Action: delayed
Status: 4.4.7
Will-Retry-Until: Sat, 18 Apr 2009 16:54:49 -0700
X-Display-Name: user2@domain1.com

Final-Recipient: rfc822;user3@domain1.com
Action: delayed
Status: 4.4.7
Will-Retry-Until: Sat, 18 Apr 2009 16:54:49 -0700
X-Display-Name: user3@domain1.com
____________________________________________________________
Reporting-MTA: dns;server.domain.com

Final-Recipient: rfc822;user 4@domain2.com
Action: delayed
Status: 4.4.7
Will-Retry-Until: Sat, 18 Apr 2009 16:54:04 -0700
X-Display-Name: user 4
___________________________________________________________
Reporting-MTA: dns;server.domain.com

Final-Recipient: rfc822;user5@domain2.com
Action: delayed
Status: 4.4.7
Will-Retry-Until: Sat, 18 Apr 2009 16:54:49 -0700
X-Display-Name: user 5


Final-Recipient: rfc822;User 6@domain2.com
Action: delayed
Status: 4.4.7
Will-Retry-Until: Sat, 18 Apr 2009 16:54:49 -0700
X-Display-Name: User 6


Final-Recipient: rfc822;User 7@domain2.com
Action: delayed
Status: 4.4.7
Will-Retry-Until: Sat, 18 Apr 2009 16:54:49 -0700
X-Display-Name: User 7


Final-Recipient: rfc822;User8@domain2.com
Action: delayed
Status: 4.4.7
Will-Retry-Until: Sat, 18 Apr 2009 16:54:49 -0700
X-Display-Name: User 8


Final-Recipient: rfc822;User9@domain2.com
Action: delayed
Status: 4.4.7
Will-Retry-Until: Sat, 18 Apr 2009 16:54:49 -0700
X-Display-Name: User 9
__________________________________________
Reporting-MTA: dns;server.domain.com

Final-Recipient: rfc822;user10@domain3.com
Action: delayed
Status: 4.4.7
Will-Retry-Until: Sat, 18 Apr 2009 16:54:49 -0700
X-Display-Name: user 10


Final-Recipient: rfc822;user11@domain3.com
Action: delayed
Status: 4.4.7
Will-Retry-Until: Sat, 18 Apr 2009 16:54:49 -0700
X-Display-Name: user 11


Final-Recipient: rfc822;user12@domain3.com
Action: delayed
Status: 4.4.7
Will-Retry-Until: Sat, 18 Apr 2009 16:54:49 -0700
X-Display-Name: user 12