1. A method for transmitting an email message to multiple groups of recipients, the method comprising: composing an email message including digitally encoded content and email addresses associated with a first group of intended recipients; adding, in response to a user request, email addresses associated with a second group of intended recipients; sending the email message to the first and second groups of intended recipients; and preventing the first group of intended recipients from determining the identities of the second group of intended recipients, and preventing the second group of intended recipients from determining the identities of the first group of intended recipients.