Method (200; 500) for securing forwarding of an e-mail between a sender e-mail server and a recipient e-mail server which are connected one to the other by an IP network, the IP network comprising at least first and second sub-networks (21; 22) which have different security levels between them, a data packet being transmitted between a pair of nodes of the first sub-network, respectively of the second sub-network, whilst guaranteeing that, at any point of the routing path of said data packet between the nodes of said pair of nodes, said data packet will be processed according to a first security level, respectively a second security level, the sender e-mail server being connected to the first sub-network by a first input access point and to the second sub-network by a second input access point and the recipient e-mail server being connected to the first sub-network by a first output access point and to the second sub-network by a second output access point, said method consisting of the following steps, produced by the sender e-mail server: - analysing the content of an e-mail to be sent so as to determine a value of a security level indicator corresponding to the security level with which to forward said e-mail over the IP network to the recipient e-mail server: - selecting, as a function of the value of the security level indicator and the first and second security levels, the first input access point or the second input access point as access point to the IP network on which to transmit the e-mail to be sent; and - transmitting the e-mail to be sent to the selected input access point, characterised in that the method comprises, furthermore, the following steps, produced by the sender e-mail server, consisting of: - determining if the e-mail to be sent has several recipient usernames; in the affirmative, - checking a database associating with each recipient username a value of a reception quality indicator corresponding to a capacity of the recipient to receive and check an e-mail in real time; and, in the case where the values of the reception quality indicators are different from each other, - modifying the e-mail to be sent so as to integrate therein a distribution indicator of the e-mail which is indicative of an asynchronous reception of the e-mail. - analysing the content of an e-mail to be sent so as to determine a value of a security level indicator corresponding to the security level with which to forward said e-mail over the IP network to the recipient e-mail server: - selecting, as a function of the value of the security level indicator and the first and second security levels, the first input access point or the second input access point as access point to the IP network on which to transmit the e-mail to be sent; and - transmitting the e-mail to be sent to the selected input access point, - determining if the e-mail to be sent has several recipient usernames; in the affirmative, - checking a database associating with each recipient username a value of a reception quality indicator corresponding to a capacity of the recipient to receive and check an e-mail in real time; and, in the case where the values of the reception quality indicators are different from each other, - modifying the e-mail to be sent so as to integrate therein a distribution indicator of the e-mail which is indicative of an asynchronous reception of the e-mail.