The PAN is not a replacement for a spam/email gateway. While it can detect certain malicious files as attachements. Take a look at the following article, it goes over dynamic block lists and some are related to known spam systems.
Also here are a few I have used in the past:
Hope that helps.
@OtakarKlier already mentioned how you could pull known spam lists; I'll add that you can also use Wildfire to look at malicious attachements. However as already mentioned the firewall isn't really the place for blocking spam as it's an all or nothing afair. If it blocks something that you want to allow, there isn't a process of ever retrieving this email or its contents like in true spam filters.
IMO there's a pretty big difference in process and protections a firewall is supposed to perform comparing that to an e-mail monitoring service. Which is why Palo partnered ProofPoint and have strong integrations with them.
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