System enabling the cancellation of previously-sent e-mail comprising a data transmission network, wherein a plurality of users are connected to the network, each user being able as a sender to forward an e-mail over the network to at least another user as a recipient connected to the network, and wherein a message transfer agent MTA is associated with each user for sending the e-mail when the user acts as a sender and delivering the e-mail when the user acts as a recipient. Each MTA includes a cancel mailbox for transmitting a cancellation message to the recipient(s) when the user associated with the MTA is a sender wanting to cancel a previously-sent e-mail or for managing the cancellation of e-mails in the mailbox of the user associated with the MTA upon receiving the cancellation message from the sender when this user is a recipient.