This is the protocol definition between the Recipient SMTP Server (and the Recipient Mail Client) and the Authentication Server.

SMTP Recipient sends VERIFY Command

<VERIFY>
   <EMAIL>aa@simplicato.com</EMAIL>
   <AUTHKEY>zxXbxkjzwL...ZIc9VGqTc</AUTHKEY>
   <SEQ_NO>93493439</SEQ_NO>
</VERIFY>

The AS replies back either VALIDKEY or INVALIDKEY

<VERIFY_REPLY>
   <EMAIL>ramt@simplicato.com</EMAIL>
   <SEQ_NO>93493439</SEQ_NO>
   <STATUS>VALIDKEY</STATUS>
</VERIFY_REPLY>