1. A method comprising, performing at an authentication service system: receiving an authentication session invitation request from a sender of an email, wherein the email is sent to a destination address of a recipient, and wherein the authentication session invitation comprises an email address of the sender and an invitation to create an authentication certificate for the sender and the email; sending, to the sender, a session identification and a text-phrase prompt comprising a text phrase; receiving, from the sender in response the text-phrase prompt, the destination address of the recipient and an audio signal; retrieving, based on the email address of the sender, a voice template associated with the sender; comparing the audio signal to the voice template, to yield a confidence score; when the confidence score exceeds a threshold: generating the authentication certificate; and packaging the session identification with the authentication certificate, to yield authentication information; when the confidence score does not exceed the threshold, packaging the session identification and the confidence score as the authentication information; and transmitting the authentication information to the destination email address of the recipient.