Email being forwarded with eml attachment

I have set up an email account on InMotion cpanel, and then a forwarder. When I send to this email address, it is received in the InMotion inbox (seen via webmail). But never gets to the forwarded address. If I go into the mailbox using webmail, I see the email. If I manually forward, it is rejected for having an eml attachment. If I send a simple email from the webmail account, it goes OK. So the problem seems to be that InMotion is forwarding by creating an eml attachment, which is then rejected somewhere. With automatic forwarding set up, I never see a rejection notice, but when I forward manually, I see the rejection.