The former, which is quite useful, when you keep mistakenly only replying to the previous respondent in a group email chain. When this becomes an issue is when

The former, which is quite useful, when you keep mistakenly only replying to the previous respondent in a group email chain. When this becomes an issue is when you don't realize what you've done; then the person you replied to might not notice it. And then they might only reply to you since it's not longer a group email... and so on.