Spam "from" reserved netblocks

Recently, I've got a lot of spam with headers forged to appear to be from 
an IANA reserved netblock. The latest was "from" 111.120.227.30, which I 
understand from Sam Spade to be included in RESERVED-8. (here's an excerpt 
from Sam Spade)

NetRange:   96.0.0.0 - 123.255.255.255                        
CIDR:       96.0.0.0/4, 112.0.0.0/5, 120.0.0.0/6 
NetName:    RESERVED-8
NetHandle:  NET-96-0-0-0-1
NetType:    IANA Reserved

I have a suspicion that anything apparently from a reserved netblock is 
probably forged, and thus probably spam. If so, I can use such netblock 
information to supplement my spam filters - but are my suspicions true?

TIA,

-- 
Geoff Lane
Cornwall, UK