It’s easy: from your Dashboard, click on “Settings” and then “Global Settings”...

... then deflag “Suppress bounce”.

If you aren’t receiving any bounce notification, try to check your Dashboard, clicking on “Report” and selecting “bounce”.

If you’re sure that the account that produced the error is correct, you can select that email account from your bounce report and put it back among the active contacts: you just have to select it and click on “Cancel selected”. Shipping to that account(s) will be possible again.