So I doubt Marketo likes that sort of thing. How exactly did you remove the Contact from the Account? Marketo will likely re-sync the record as a Lead now.
Email Address is the unique key for Marketo. It doesn't inherently block leads with no email address, but SFDC might and it's bad to do this to Marketo. Did you null it out before the sync error, or after?
Appreciate your time & apologies for not specifying the CRM Type.
I use Microsoft dynamics CRM 2013 on premise.
As far deleting the lead goes - The account is a lookup field on the Contact Record - I just set it to a blank field.
I have never testing separating a contact from an account. But for the second point I am sure (tested and retested) that the sync between SFDC and Marketo is base on the SFDC ID. Therefore, you can easily wipe out the email address in SFDC and it should sync with Marketo.
If it does not, I suspects it's because you have a sync filter that prevents the contacts without an email address to sync with Marketo, either directly, or through a workflow in SFDC and a "Sync with Marketo" field (or similar).
I Microsoft Dynamics CRM 2013 On Premise.
It makes complete sense that Marketo would sync the lead based on SFDC ID & not sure if its safe to assume that for MSD CRM it would use Marketo Unique Code.
We did run a workflow for the first time partial sync to set the "SynctoMkto" flag to yes for users with email addresses only. But that was a one time workflow - will go back and check if its still active.
Appreciate your time.