Thanks for the quick feedback. Our main problem is the poor quality of the current data in SFDC. There are many duplicates on lead and contact level and we actually would like to start using Marketo without first cleaning the existing data in SFDC. That would set us back weeks or months. It all has to do with the way SFDC is being used in our organization. We work with a network of partners / resellers, and they enter their own Leads into our SFDC, which in turn can turn into Opportunities + Contacts + Accounts. We try to deduplicate double Accounts when entered by them, but the leads / contacts from our partners co-exist more or less next to our own in the same system and duplicates are a reality we have to live with in SFDC. This is set up like this mainly for sales, so sales has pipeline insights. From a legal point of view, we also cannot target our partners' leads / contacts as we have no optin for that. So that's why I'm looking for a way to have our own leads go into SFDC without interfering with those from our partners - even when they for example share the same email address (is that's possible at all)...
There is one thing that works a bit in our advantage I think: the current email list we just imported in Marketo is deduped with the contents of our SFDC (so email addresses currently exist in one of the two systems only).
I currently don't quite understand how the link betweens Marketo and SFDC works once the sync is ON. Is the email field the main player for the sync? Or is there alwasy a one-to-one relation between records in the two systems? I've been trying to find some basic information on this but haven't found it yet. Any info / links on this topic are appreciated.