I propose killing MailSession's "interactive" flag, along with the corresponding get/set functions?  The flag no longer seems to serve a purpose.  In the current code base, it's always TRUE.

Based on CVS history and mail/ChangeLog.pre-1-4, it looks like the code dates back to 2000, and its original purpose was to delay asking the user for a password until after the start-up splash screen disappeared.

Here's the oldest relevant ChangeLog entry that I could find:

2000-12-11  Dan Winship  <danw@helixcode.com>

        * session.c (mail_session_enable_interaction): New function to
        tell the code that it's ok (or not) to interact with the user
        when trying to authenticate to a service. Starts out turned off.
        (mail_session_request_dialog): If interaction is disabled, fail
        if the password isn't in the cache.

        * component-factory.c (owner_set_cb): Call
        mail_session_enable_interaction() after everything else. (This
        means that the IMAP password dialog will no longer pop up [under
        the splash screen] at startup.)