I corrected this by adding a generic last name for everyone on the list. I did that yesterday but none of the leads have synced. Is there an action I have to take to "re-sync" since the email blast has already gone out?
In the Marketo admin section under SalesForce, you can edit your sync options so that Marketo will automatically check for required SFDC fields and put a value there.
I use separate centralized life cyclesmart campaigns to process the sync (never in programs and campaigns). Within those smart campaigns I check if the mandatory fields are present. If not, they are populated with "N/A".