It would be really helpful if Marketo had a flag or notice calling out any email addresses in lists at the campaign level that are not leads in Marketo - aka those folks won't receive the email because they don't yet have a record in the Lead Database. The only way to check currently is downloading and cross-checking through VLookUps - unless I'm missing something?
If I understand well, you want to know what leads are in a file and are not in Marketo without importing this file to Marketo ?
But how would Marketo know ? It has no divination gift
The only way to do this is to import the file in a Marketo list and then check which leads were created during import. For this, you can use a smart list like this :
What you can try is to add a Filter in the Smart List that is 'Email Address'. Then you add all the email addresses on your list to the filter (there are restrictions to the number allowed here). Looking at the leads in the Smart List you will only find the leads that are in the MKTO database.
Downloading this list you'd still need to dedupe against the original file. You could do this by adding it to the original list and using a 'countif' formula in Excel. This way you'd count all the occurences in an array. All the 1's will not be in MKTO.