What to return if a DNS lookup fails?

I suggest following return values:

If the DNS lookup returns       NXDOMAIN        no data         error
then return a result as in following table:

get SPF record                  fail (or none)  none            error
get included SPF record         unknown         unknown         error
get redirect SPF record         unknown         unknown         error
a                               unknown         no match        error
mx                              unknown         no match        error
ptr                             no match        no match        error
exists                          no match        no match        error

What do you think?

Roger