These are the capabilities that the plugin defines:
// Capabilities used by the plugin
define( 'MAILUSERS_EMAIL_SINGLE_USER_CAP', 'email_single_user' );
define( 'MAILUSERS_EMAIL_MULTIPLE_USERS_CAP', 'email_multiple_users' );
define( 'MAILUSERS_EMAIL_USER_GROUPS_CAP', 'email_user_groups' );
define( 'MAILUSERS_NOTIFY_USERS_CAP', 'email_users_notify' );
This is the code from the actual function that assign the capabilities to the standard WordPress roles.
function mailusers_add_default_capabilities() {
$role = get_role('contributor');
$role->add_cap(MAILUSERS_EMAIL_SINGLE_USER_CAP);
$role = get_role('author');
$role->add_cap(MAILUSERS_EMAIL_SINGLE_USER_CAP);
$role->add_cap(MAILUSERS_EMAIL_MULTIPLE_USERS_CAP);
$role = get_role('editor');
$role->add_cap(MAILUSERS_NOTIFY_USERS_CAP);
$role->add_cap(MAILUSERS_EMAIL_SINGLE_USER_CAP);
$role->add_cap(MAILUSERS_EMAIL_MULTIPLE_USERS_CAP);
$role->add_cap(MAILUSERS_EMAIL_USER_GROUPS_CAP);
$role = get_role('administrator');
$role->add_cap(MAILUSERS_NOTIFY_USERS_CAP);
$role->add_cap(MAILUSERS_EMAIL_SINGLE_USER_CAP);
$role->add_cap(MAILUSERS_EMAIL_MULTIPLE_USERS_CAP);
$role->add_cap(MAILUSERS_EMAIL_USER_GROUPS_CAP);
}
Hope that helps narrow down the problem.