ARCHIVED: What does the "Address family not supported by protocol family, host unknown" error mean when email is returned to the sender?

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It is a name server error, but not necessarily from the local name server. The error (and bounce) could result from any name server the message encountered along the delivery route.

The simplest solution is to wait overnight and try resending the message.

Here is an example of the error:

  ----- Transcript of session follows -----
  421 vuse.vanderbilt.edu (tcp)... Deferred: Address family not supported 
  by protocol family
  554 smithj@vuse.vanderbilt.edu... 550 Host unknown (Non-recoverable name 
  server error): Address family not supported by protocol family

If you encounter this error, try waiting awhile, then from a Unix host, enter:

  /etc/ping host.domain

Replace host.domain with the appropriate hostname and domain, for example:

  /etc/ping vuse.vanderbilt.edu

This will tell you if the node is currently reachable. If it is, try resending the message.

Note: On some Unix systems, the ping command will not be located in the /etc directory, but in a different directory, such as /sbin. If so, the above commands will have to be modified so that /etc is replaced with the correct path to the ping command. To learn how to locate the ping command, see the Knowledge Base document Find the correct path to an executable file in Unix