How can I mark as spam mails with missing "From" ?
A little rule:
header __FROM_PRESENT exists:From meta MISSING_FROM !__FROM_PRESENT score MISSING_FROM 1.0 describe MISSING_FROM from header missing
Note: use with caution, as I've not done any testing of this.
However, the rule works the same way MISSING_SUBJECT works in the standard 3.0 code, and I've done plenty of similar rules before.