All my spam is "Undeliverable", What's up?

Lately, all the spam I receive seems to be undeliverable messages,
ie messages I did not send that are coming back to me as undeliverable
mail or failure notices.

These returned email error messages indicating a message or messages you did not send are undeliverable are not spam but the original message that was undeliverable was sent by a spammer.

This indicates a spammer used/forged your email address to appear as the sending email address for a bulk spam mailing or mailings.

For most email accounts, there is nothing preventing a user from providing any email address to appear as the sending email address for an account which is entered in the Email Address field for the account preferences or by entering a different Reply To address in the message header.

Apple prevents doing the first with a .Mac account and although there may be some other email account providers that have the same restriction, spammers don't use such email accounts to send their garbage.

Since your email address is likely included in a spammers "known good" email address list, he/she can pull a random email address from this list to appear as the sending email address for a bulk spam mailing with the sending address constantly being changed or a spammer has taken over a Windows PC remotely and is using the email client on that PC to send out bulk spam mailings and your email address is included in the email client's address book.

Unfortunately, there is nothing you can do about this.

So even "bouncing" these emails won't help, will it?

No and bouncing a return email error message received from an incoming mail server indicating a sent message was not accepted or deliverable won't accomplish anything.

Since most or all spammers don't use a valid sending or return email address, using the bounce feature on spam is a complete waste of time since all you get for your efforts is a return email error message from an incoming mail server indicating the bounced message was not deliverable to the recipient or the email address is not valid for that domain as you have been getting now for messages you did not actually send.

And if a spammer uses a valid email address that was forged like yours (the same has also happened to me and to many others), using the bounce feature on these messages will work but the bounced message will be sent to some unsuspecting soul who like you did not send the spam to begin with so the bounce doesn't make it to the intended target.

If a bounced message does happen to make it to the intended target spammer, these guys have ways to determine that your email address is valid from the long headers of the received message.

Long story short, do not use the bounce feature with spam.

There should be a law or something.

FWIW, there are lots of laws about this in lots of countries. The problem is enforcement -- a spammer using a server based in a country with little regulation often falls into a gray area of jurisdiction, & even when it doesn't the chances of apprehension are low unless there is significant loss due to fraud to motivate authorities.