An email that you regret sending.

The regret felt after you realize that you should not have provided your email address to someone who then sends various annoying and otherwise intrusive emails.

e-gret (noun, verb): The feeling of sorrow, shame, remorse, or fear one experiences after sending an ill-advised electronic message (e-mail, IM, text message) that cannot be recalled.

A message of regret sent by electronic mail. (defined by E Delk)

aptly describes the moment of immediate regret when one ventures into the comments section of any news piece.

a regret you have about something that happened over the internet