[BlueOnyx:17631] Mail Delivery Subsystem error mesage

[Tom]

Since rebuilding my server, I've been getting this Mail Delivery Subsystem 
error message several times per day. Is there a simple fix?
Thanks in advance
Tom


The original message was received at Wed, 20 May 2015 02:01:11 -0400
from localhost [127.0.0.1]
   ----- The following addresses had permanent fatal errors -----
<admin at localhost.localhost>
    (reason: 550 Host unknown)
   ----- Transcript of session follows -----
550 5.1.2 <admin at localhost.localhost>... Host unknown (Name server: 
localhost.localhost: host not found)