What does is mean if an email is blocked?

Blocks are similar to bounces, in that they are logged when an e-mail could not be delivered to a recipient address. However, a block is only logged when a delivery failure occurs due to a temporary problem, such as the recipient email server being down, or if the mailbox is full. Block events do NOT cause an e-mail address to be flagged as invalid, and therefore can still be sent to in future deployments.