Apply this rule after receiving the message
If From does not contain "@"
[Permamently] delete it (or whatever action you want)
stop processing more rules
This is not verbatim but you should be able to find the appropriate
clauses to use in this rule. If there is no "@" character in the From
field then it has no e-mail address listed in it. A blank From field
would obviously not have any characters so it would also not have the
"@" character.