SPF record for the domain is configured similarly to this example:

sub.domain.com   text =        "v=spf1 MX ip4:1.2.3.4 ip4:1.2.3.5 -all"

It's noticed that Symantec Messaging Gateway (SMG) for this domain generates spf=unknown verdict.

Case sensitive handling of SPF mechanisms.

Symantec is currently investigating this. In the meantime please use lowercase letters to define SPF records, as example change "MX" to "mx" if it's the case.