Hi.
My guess is that this is Data ONTAP 7G (releases 7.x and earlier). If so the "User Unknown" is reported in a variety of situations, one of them being when a SMTP RCPT TO: command is sent to a mail server and a 55x code is returned. There can be others.
I suggest using an IP address instead of a hostname for autosupport.mailhost and to use "telnet" to that IP address using port 25 to verify whether the mail server will take the specified mail addresses. The sequence would be something like
telnet xxxx 25
HELO myhostname
MAIL FROM:<Postmaster>
RCPT TO:<autosupport@netapp.com>
Any 4xx or 5xx is an error and means the SMTP server is rejecting it.
Domain Name Serrvice (DNS) and other host directory services can map differently depending on a host's configuration. This is why I suggested using an IP address.