Error looking up SPF record

In section 3 of the specifications it says:

     Error: indicates an error during lookup; an MTA MAY reject the
     message using a transient failure code, such as 450.

I think "MAY" should be replaced by "SHOULD". Otherwise a spammer could
simply use a domain name (like bunita.net, elevation-tour.net or eguo.com)
where looking up the TXT record fails, and the MTA's would accept the spam.

Roger