How can I append a disclaimer to outgoing messages?

Disclaimers are best appended in filter_end, after all message parts have been processed. append_text_boilerplate handles plain-text parts and append_html_boilerplate handles HTML parts, inserting the HTML disclaimer before the closing </body> tag when possible.

use Mail::MIMEDefang::MIME;

sub filter_end {
    my($entity) = @_;

    # Only append disclaimer to outbound mail
    return unless $RelayAddr =~ /^(192\.168\.|10\.|172\.1[6-9]\.|172\.2\d\.|172\.3[01]\.)/;

    my $text_disclaimer = "\n--\nThis message and any attachments are confidential.\n";
    my $html_disclaimer = '<br><hr><p style="font-size:small;color:#666">'
        . 'This message and any attachments are confidential.</p>';

    append_text_boilerplate($entity, $text_disclaimer, 0);
    append_html_boilerplate($entity, $html_disclaimer, 0);
}