An email block means an ISP has rejected your message as it may think that you are spamming them. Please click here to see the reasons for blocking (which are the same as for bulking).
Example
Blocking is considered to be more severe than bulking. For example, if a community mails to a moderate level of Invalid Email Addresses at Yahoo, then Yahoo may bulk subsequent messages by that sender (community). However, if that community mails to a very high number of Invalid Email Addresses, then Yahoo may decide to bounce those emails as a block. In such a scenario, those messages that have bounced did not reach the reader.
Both bulking and blocking contribute to low response rates.
Some ISPs temporarily block for a set time (ie. 24 hours), after which time they lift the block automatically. However, there are other ISPs that issue a permanent block, which would require EDM to escalate to the ISP in order to resolve the issue.
More Information
If you have very low response rates and think you may have a bulking or blocking issue, please click here for remediation steps.