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
know its long time since you asked the question ... but we just got the same issue...
for the "unknown user" check the mail transport server (all if you have many) for mail forwarding and/or check if any firewall port 25tcp used for the outbound mail.
The AutoSupportStatus OFF is because the filer can't send the asup to firstname.lastname@example.org. If the mailing would do, send a options autosupport.doit WEEKLY_LOG and give "now.netapp.com" a bit of time to refresh.