Is the delivery rate the % of leads that were delivered OR the % of leads that were delivered to the inbox (did not land in the junk folder)?
If it is the former, then is there any way, using Marketo or other tools, that I can find out my Inbox Delivery Rate (the % of leads where my email did NOT land in a junk/spam folder)?
It is the first one,
Sent - HB - SB = Delivered.
You can use Marketo's Deliverability tool (250ok) to run campaign tests to get an approximate idea of your actual inbox hit rate. Other tools also exist.