"no data known" vrs "host not found"

[admjcd]

When ever we send mail we get back these NDRs when ever the domain is not correct. Sometimes we get them when the domain is correct. Both types are "host unknown".  One says  "no data known" and the others say "host not found" .  What is the difference between these two and why should we get the "no data known" intermittenly message when the domain does exist?
 
 
22-Mar-2002 10:11:41  Action: Message Accepted  Client: 152.122.15.110  To: 
Subject: Returned mail: Host unknown (Name server: vpndymanics.com: host not found)  Size: 2827  SMTP ID: M2002032210114132056  

22-Mar-2002 11:34:41  Action: Message Accepted  Client: 152.122.15.110  To:   
Subject: Returned mail: Host unknown (Name server: atc.army.mil: no data known)  Size: 8490  SMTP ID: M2002032211344115470  


22-Mar-2002 12:16:33  Action: Message Accepted  Client: 152.122.15.110  To: 
Subject: Returned mail: Host unknown (Name server: soucecode.com: host not found)  Size: 2570  SMTP ID: M2002032212163325616  

22-Mar-2002 12:20:03  Action: Message Accepted  Client: 152.122.15.110  To: 
Subject: Returned mail: Host unknown (Name server: worldnet.att.net': host not found)  Size: 4033  SMTP ID: M2002032212200329724  

22-Mar-2002 12:51:23  Action: Message Accepted  Client: 152.122.15.110  To: 
Subject: Returned mail: Host unknown (Name server: hua.army.mil: no data known)  Size: 41681  SMTP ID: M2002032212512214595  

22-Mar-2002 12:54:36  Action: Message Accepted  Client: 152.122.15.110  To: 
Subject: Returned mail: Host unknown (Name server: hua.army.mil: no data known)  Size: 11271  SMTP ID: M2002032212543620425  

22-Mar-2002 14:03:39  Action: Message Accepted  Client: 152.122.15.110  To: 
Subject: Returned mail: Host unknown (Name server: adf.as.sdff: host not found)  Size: 3200  SMTP ID: M2002032214033928261  

22-Mar-2002 14:36:03  Action: Message Accepted  Client: 152.122.15.110  To: 
Subject: Returned mail: Host unknown (Name server: attbi.com,: host not found)  Size: 87882  SMTP ID: M2002032214360328071  

22-Mar-2002 14:52:21  Action: Message Accepted  Client: 152.122.15.110  To: 
Subject: Returned mail: Host unknown (Name server: mail1.qrs.com.: host not found)  Size: 15917  SMTP ID: M2002032214522032482