Missive 10.64.1 could be downloaded from the developer's website when we last checked. We cannot confirm if there is a free download of this software available. We recommend checking the downloaded files with any free antivirus. Missive lies within Communication Tools, more precisely E-mail Tools. This program was originally produced by Missive.
From the developer:
Missive is a team inbox and chat tool that empowers teams to collaborate around email. It provides a modern interface where you can create your own team add team members to allow access to email inboxes. Each member can either me a "member" or "observer". You can discuss ideas with everyone, teams, or specific teammates using chat rooms. You can also do one-to-one chat with visitors on your website.
You may want to check out more software, such as SmartPOP2Exchange, SDP or Cloudmark Desktop for Microsoft Outlook, which might be related to Missive.