For more information on this and other mail encryption settings, see the topic Editing the Configuration Settings document for Domino Web Access.
If SSL connection is required for either the client or both the client and server, Domino Web Access users cannot read or send encrypted messages when connected via HTTP. If the user is connected via HTTP, they must switch to HTTPS when accessing the encrypted message on the server. This switch occurs automatically when sending encrypted mail. The user will be prompted to switch when reading encryped mail.
Deployment differences between Notes and Domino Web Access