Forwarding E-mail

It is permissible to forward an e-mail without first obtaining permission to do so? From The Jewish Ethicist:

The moral of the story is that we should be very careful not to forward emails unless we are sure the sender approves. Even when it is clearly appropriate to transmit the message to others, consider if it may not be sufficient to provide a brief summary of the sender's words, rather than just forwarding the exact words. Likewise, even when forwarding someone's exact words, don't forward the whole letter if the really relevant section is brief.

For more on the cherem of Rabbenu Gershom, see this post.