One variation of a method for end-to-end encryption of electronic mail includes: receiving an email encrypted according to a first encryption protocol and designating a recipient within an external domain; verifying encryption protocol supported by the recipient's mail client; in response to a recipient exclusion database identifying the recipient, encrypting the email to a less-robust encryption protocol supported by the recipient mail client and transmitting the email to the recipient; in response to the recipient exclusion database excluding the recipient and the recipient mail client supporting the first encryption protocol, transmitting the email encrypted according to the first encryption protocol to the recipient; and, in response to the recipient exclusion database excluding the recipient and the recipient mail client not supporting the first encryption protocol, generating a notification email including a hyperlink to a secure webpage containing content of the email and transmitting the notification email to the recipient.