I have a list of invalid emails. I ran a report to cross reference these with people who have opened an email recently, and there are some that are marked as invalid emails but have had some activity.
So are invalid emails non-existent (or no longer existing) emails that can be deleted?
A hard bound renders an email address invalid in Marketo. So in this case perhaps there was some activity at one time, but the email address no longer exists
yes as long as you are sure that there is no useful information such as valuable info in your CRM. Furthermore, if you delete the lead from Marketo but you keep it in the CRM, it will come back. So if it's a lead in SFDC or a prospect in MSD, you may delete it. If it's a contact, it is usually a bad idea.
A good way to deal with this is to erase the invalid email address in the CRM. The lead will remain in the system but without an email address. IN case that person comes back later with a correct email address, you will still be able to merge the old one with the new one and it will not count against Marketo licence fee since there is not email.
Email Invalid=T is the way to look at this. Are you sure this is the field you used?
There are several docs on how this works and what to consider: