Okay, what mail server are you going through? Putting something between parentheses after the email address is breaking all known email RFCs. I'm surprised that your message doesn't get rejected before it arrives at SG.
I tried that and indeed, I reproduced the problem. I sent an email to an SG account from a trusted sender
foo@bar.net and it arrived with the correct tag "(
foo@bar.net: trusted sender for your account)". Then I tried to send the same email again, but this time with
From:
foo@bar.net (blah blah)
and sure enough, the sender wasn't recognized as trusted.
I'm not sure we can call this a bug. Look, you're not allowed to put comments like this in the From field. The only way allowed by the RFC is to put these comments before the address, like this:
From: "reply from Jane" <your_guestbook@foo.com>
That's legal. Whereas
From:
your_guestbook@foo.com (reply from Jane)
is not legal.
Does that make sense? Can you fix the From field in the sender?