What does Unknown mean on the bounced reports?

That response occurs when the email server sending us the bounceback message does not provide a reason - meaning all it reports back to our servers is that the email is not deliverable, and will never be. This could be due to a spam filter or other corporate email filter setting.

In these cases, we recommend switching the contact to plain text format and resending the campaign to see if it can be delivered. Plain text, while not as visually impactful, is the simplest form of email and is almost always not a problem with spam and corporate email filters.