I actually dug up the exchange with Customer Service from a couple of years ago. This is an extract of what they told me:
There are a number of reasons why someone can be on our suppression list, and it’s generally a user-by-user basis and not a global thing. The best thing to do is to reach out to us in these cases so we can take a look, and we can remove any users who are on the list.
[…]
Generally if an email is on our suppression list, it’s because the account is really newly setup, or emails from Coda in the past have bounced and/or been marked as spam too many times.
My company also uses Microsoft Outlook and I suspect the issue was that for whatever reason, during the first 24 hours upon creation, the email address was only able to receive internal emails. I created the coda accounts straight-away and that probably caused the bouncing and the inclusion of the accounts on this ‘Suppression list’.