It depend on how you have set up the rule to sync leads over from Marketo to SFDC. I like to have a single sync rule to manage this so that is it easy to diagnose any problems that come up.
You should identify the minimum information that it takes to route the leads to the right sales rep. Otherwise you will be syncing leads over that will be orphaned. That's ok if you have a way to manage this over in SFDC.
Leads are synced based on a trigger campaign of lead is created AND has First Name, Last Name, Email Address, Company, Country, Zip Code, and has a tiny number of points ie 5 on a 100 pt max to ensure a heartbeat and not a spambot. Also this way you can control uploading of purchased lists.
I require country so that I exclude EU and Japan countries from purchased list uploads.
Typical list purchases only allow one time use. If this is the case, I would NOT sync to SFDC. I would just upload the email addresses. Put them through a program where I can capture their name so they are now mine, and then write the rules to purge the addresses that were not captured at the end of the campaig based on create date and some other logic.
If you have a purchased a list and you own the names then the above does not apply.
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