How to add a new field mapping in Marketo?

Apoorva, as Email Invalid is standard field, you can not map directly. Instead update another custom field based Email Invalid data and send the custom field info to SFDC.

As Kapil said, you have to create a new custom field in SFDC that syncs to Marketo, then populate that field with the same information that Marketo has for email invalid. This can be done using 2 smart campaigns.

1 (Batch) Smart list: email invalid = True; Flow: Change Data Value: [your new email invalid field] = true

2 (Triggered) Trigger: data value changes = email invalid (new value: true); Flow: Change Data Value: [your new email invalid field] = true

There's also a third campaign that I would advise, so that you can update email addresses and remove them from email invalid status:

3 (Triggered) Trigger: data value changes = email address, Filter: email invalid = true; Flow: change data value: Email invalid = false, change data value: [your new email invalid field] = false

I would also advise making this a field that cannot be updated in SFDC. Let Marketo handle the "unchecking" of the field by an actual email address change, not just someone unchecking the email invalid field.