I don't know why the server doesn't accept those attachments, but the attachment the server is complaining about is the very message you're trying to forward because you're forwarding the message as an attachment. It doesn't matter whether the forwarded message has attachments of its own or not. The default forwarding mode can be changed in <b>Settings > Composition</b>. You may choose there to forward messages <b>Inline</b> instead of as attachments. You can also choose how you want to forward a particular message regardless of the default setting by right clicking on the message you want to forward and choosing <b>Forward As</b>.