Some email software insists on sending forwarded mail as an attachment instead of within the body of the message. Many servers will not accept messages with this file type (usually filename.eml or something similar.)
An example of this sort of bounce message is as follows:
>This message was created automatically by mail delivery software.
>
> A message that you sent could not be delivered to one or more of its
> recipients. This is a permanent error. The following address(es) failed:
>
> username@recipient_domain.com
>
> This message has been rejected because it has a potentially executable attachment “ForwardedMessage.eml”
> This form of attachment has been used by recent viruses or other malware. If you meant to send this file
> then please package it up as a zip file and resend it.
You will need to make sure your mail program is configured to send forwarded mail as “Inline” and not as an attachment.
In Outlook: Tools-> Options-> Preferences Tab-> E-mail Options-> When fowarding a message (make sure it isn’t set to “Attach original message”)
In thunderbird: Tools-> Options-> General-> Forward Messages-> select “Inline”
