Sure - SPF may very well only check the envelope sender, but isn't that usually plenty? If you can't spoof the Linkedin envelope sender, and the mail goes to SPAM, who cares if the From: address is spoofed? The user likely won't see it anyway.
Anybody that can run sendmail as root (or use an alternate program, or compile their own) can spoof the envelope sender.
Ah, gotcha. You're saying that you sent an email with a valid envelope sender from your domain, with only a spoofed From: address, and gmail sent it back. If so, nice!