[MitziMoto]

> is it really true that hitting the "Spam" button on a site like Yahoo or Gmail informs the sender of your email address?

Yes. The feedback loop shows the sender who marked the mail as spam.

> What is a user supposed to do in the case of an accidental bump of the "Spam" button? Is there really no way to undo the damage (both to the sender and to the willing recipient)?

There is no good solution, it kind of sucks for both the sender and the receiver. When you click that spam button, intentional or not, I can no longer send you email until going through a sometimes laborious process of working with the email service to get the email unblacklisted. If you really want to receive email again, send an email to the company/person and let them know you accidentally clicked the spam button so they can work it out with the email provider.

Gmail handles this the best IMO as it gives the user ~5 seconds to click "undo" before reporting the spam. As far as I can tell most other email clients are instantaneous.

[Steuard]

That's maddening. The really frustrating part is that this effect (and especially the "can't undo" part) is completely invisible to the user. Like I said, my wife missed out on some rather important messages about an upcoming event that we were attending because of this. (After a couple of months, when the event was a few weeks away, I believe she got a message from the organizers letting her know that they hadn't been able to contact her but did she really think they were sending spam. Much frustration ensued.)