Manual forward as attachment vs Auto Forward as Attachment

Having a strange issue. .....

We use Salesforce, and are trying to setup a app to automatically handle our email from Groupwise to Salesforce. The app is iHance.

For incoming mail to the GW mailbox, they said to setup an auto forwarding rule to forward as an attachment to a specific
address at iHance. So we can definitely see the mail being auto forwarded by looking at the sent mail.

BUT, if we use an automatic rule to forward incoming mail to the attached address, the attachment received at iHance (which is supposed to be the GW mail message) is blank or empty.

HOWEVER, if we manually forward the inbound message as an attachment to the attached address, the attachment received is there and readable. Their debugger shows there is clearly an EML file on the manually forwarded message.

What would cause the auto forward to send empty attachments? Is this due to enabling the flat forwarding switch on the SMTP settings, or something else?

Any one have an idea here? We've been troubleshooting this for over a week.

Kind regards,

Val