So here's the case: a lead registers on the website (and gets synced to Marketo) with a fake email address. Marketo tries to deliver him an email, the email keeps bouncing (since this email address doesn't exist) and then Marketo marks this lead as "Email Suspended", error 550 5.7.1: Relay access denied. Next time, the lead fills out form with their actual email address and is merged to the first lead. The weird part is, this new lead still gets suspended after the merge, now when it has a valid email address; their old fake email address appears again in the Email suspended cause.
What sort of magic should I apply to stop Marketo from suspending this lead?
Are you merging these leads manually? Marketo should not automatically merge the two records just because there are two form fill outs in the same browser.
The suspension will persist for 24 hours after a spam-related bounce regardless of any changes to the record. Just send again 24 hours after the suspension.
Can you show us a screenshot? Once a lead is Email Suspended, this condition persists on the record after 24 hours EVEN THOUGH Marketo will send to it as long as Email Invalid=F and Unsub=F