Senaca's thoughts on anger came just prior to the birth of Christ so I think we can safely rule out the possibility that he was talking about the internet. But I will and specifically my opinion about expressing anger in email.
Don't do it.
Make a corporate law that says any words that can in any way be interpreted negatively must be spoken rather than written. Senaca had something else in mind when he said the best remedy for anger is delay but in this day of instant communication, forcing verbal communication coupled with the delay that comes from not sending an email is remedy for all.