Changing the default preference behavior in Mozilla Thunderbird
Default forwarding preference in Mozilla Thunderbird. First, go to Edit – Preferences then you need to go to the Advanced tab on the Preferences dialog. And press the Config Editor button to start up Mozilla Thunderbird configuration editor. In the Filter box, type in forward until you see the key.
The default value, 0, means that Mozilla Thunderbird will send e-mail forwards as attachments. Changing this to 2 will inline the forward message so that the e-mail gets out to anyone who has a provider with strict mail attachment rules.
Now e-mail forwards will not bounce!