I don't know of any for spam score. Once you send an email to a lead, if the email hard bounces, the Email Invalid field is marked as "True." This is the email server telling Marketo the email is undeliverable. You should regularly audit emails marked as invalid. If you'd like to test if an email is valid, you can check them one at a time here: http://verify-email.org/
For bulk emails validity checks, I'd take a look at NetProspex or RingLead or StrikeIron.