While we definitely have duplicates, it’s hard to believe that it would cause such a huge drop in sends.
There's no reason to guess or believe one way or the other! It's a mathematical question. How many duplicate emails are there in this list?
Since you've been talking to Marketo Support, you probably already checked these things, but here are some suggestions:
- Check your SPF and DKIM statuses (under Admin>Email) to see if they are verified.
- Check to make sure your domain didn't get blacklisted (Email Blacklist Check)
- Check your communication limits (often overlooked)
I would also pull a list of who received the email in a past send (with the 140k delivered), but has not received one of your latest emails (80k delivered) and spot check a bunch of them to try and find a pattern.