A method of verifying to a recipient of an email that a sender of the email possesses a mobile telecommunications device associated with a specific telephone number, the method comprising: at the sender, sending an identifier of the email content and the telephone number to a server via the Internet; at the server, receiving the identifier and the phone number from the sender, generating a verification code, sending the verification code to the sender via the Internet, and storing the verification code with the identifier and the telephone number; at the sender, receiving the verification code from the server and returning the verification code to the server in a Short Message Service message sent by said mobile telecommunications device; at the server, confirming that the received verification code and the telephone number from which the Short Message Service message originated match the stored telephone number and verification code, and then using the telephone number and the identifier to generate an authentication string, and sending the authentication string to the sender via the Internet; at the sender, receiving the authentication string from the server, and sending the email including said authentication string and said telephone number to the recipient via the Internet; at the recipient, receiving the email, and forwarding the authentication string, the telephone number and the identifier of the received email content to said server; and at the server, receiving the authentication string, the telephone number and the identifier, generating an authentication string using the received telephone number and identifier, confirming that the received authentication string is the same as the generated authentication string, and sending confirmation to the recipient. at the sender, sending an identifier of the email content and the telephone number to a server via the Internet; at the server, receiving the identifier and the phone number from the sender, generating a verification code, sending the verification code to the sender via the Internet, and storing the verification code with the identifier and the telephone number; at the sender, receiving the verification code from the server and returning the verification code to the server in a Short Message Service message sent by said mobile telecommunications device; at the server, confirming that the received verification code and the telephone number from which the Short Message Service message originated match the stored telephone number and verification code, and then using the telephone number and the identifier to generate an authentication string, and sending the authentication string to the sender via the Internet; at the sender, receiving the authentication string from the server, and sending the email including said authentication string and said telephone number to the recipient via the Internet; at the recipient, receiving the email, and forwarding the authentication string, the telephone number and the identifier of the received email content to said server; and at the server, receiving the authentication string, the telephone number and the identifier, generating an authentication string using the received telephone number and identifier, confirming that the received authentication string is the same as the generated authentication string, and sending confirmation to the recipient.