With some hesitation, I would consider the Auto-Submitted:
header standardized in RFC3834 which allows you to suppress bounce messages for automatically generated email. As such, it's not really intended to be used on forwarded messages, but it's worth a shot.
You would add that to the forwarded message something like this:
:0 | formail -I "Auto-Submitted: forwarded" \ | $SENDMAIL -oi -f '' yourinbox@gmail.example.com
The forwarded
value is not standardized, but the way I read the spec, any value other than "no" should suppress bounces.
I also threw in the -f ''
option as a fallback; if it works, it could be useful by its own, as many mail servers will not generate bounces for messages which have an empty envelope sender. However, not all sites allow you to control this.