odoo/addons/mass_mailing/static/src/css/mass_mailing.css

.openerp .oe_kanban_view .oe_kanban_mass_mailing_campaign {
    width: 280px;
    min-height: 141px;
}

.openerp .oe_kanban_view .oe_kanban_mass_mailing_campaign .oe_kanban_header_right {
    float: right;
}

.openerp .oe_kanban_view .oe_kanban_mass_mailing {
    width: 280px;
    min-height: 141px;
}