When does Marketo determine an email address is invalid? I assume this is once an email is sent and can't be delivered but want to clarify. When a list is uploaded, some leads cannot be uploaded because of errors in their address, so Marketo does some level of email validity check upon import.
Marketo don't do any email verifications while uploading the contacts.
The errors that Marketo points out while uploading a list are if they have space in the email or any undefined character.
There are few Lauchpoint plug-in that you can use for email verifier.
I hope this answers your question.