When an email account to which you're sending produces a bounce (most of the times, because it's non-existent), our system puts it automatically in the "Suppression List" folder.

Hence, all future emails addressed to this account (and all others which has produced a similar error) won't be actually sent - and labeled as "dropped" by our system.

This happens because keeping on sending to non-existent addresses is utterly useless, and harms both your delivery rate and our servers.

That said, if you need to deliver anyway your message to a dropped account (for instance because it's existent and it's been dropped because of a temporary issue), you can do that simply erasing it from the Suppressions list.