E-mails with bracketed To addresses

[#1]

Since that unfortunate Plusnet security problem a couple of months back, I have been receiving e-mails addressed to me at "penneck@penneck.plusnet.com". I use Mailwasher to look at my incoming e-mails so I can delete any I dont like the look of. I set the filter on Mailwasher to set to delete any e-mails with this "To" address. Then I started getting e-mails addressed to "<penneck@penneck.plusnet.com>", so I had to add a filter for that too.
Now I have received an e-mail that has a "To" address like the second one above, but instead of "penneck@...." between the < and >, it has the senders address. Assuming that it is what it seems, it is an add from a local hotel, so shouldn't be a problem, but my questions are:-
1. Is this a clever (and legal) way of sending an e-mail out everywhere without needing to know the destination address?
2. Is this a way of getting a bug onto someones pc without giving away the senders real identity?
3. Is there a way of stopping this type of e-mail without having to keep adding more and more filters?
Thanks for any help
Graham P (which is apt because it can be pronounced Grumpy, and I'm not very happy about these e-mails)

[#2]

In answer to your questions
1 you can send  out emails with random address and hope one hits the target  or is caught by a catch all account. So they may send to a domain fred.com and fire off hundreds of random names like fred@fred.com
2 You can take control of other peoples pc and send out bugs virus etc via their pc without a trace (they often delete any evidence while doing this) You can send it out via any open relay server and it would be hard to trace.
3 Its a constant battle to stop spam and there is no real answer as spammers change tactics every so often to stop detection. I assume you have turned on Plus net spam filter which will get a lot of them as they check domain names so any known spammer will be filtered out.
Turn off your catch all email box as this bounces back any mail not going to a known mail box.
After that its down to your own mail filters and rules

[#3]

Thanks Samurai. I've done that. Now to see the difference.
Regards
Alotless Grumpy (if you remember the ending of my original post).

[#4]

As James_H said, to protect subscriber's email addresses when sending a message to a mailing list the proper procedure is to send the message to oneself and put the mailing list name in the "bcc:" field (which is hidden).  I think the full "To:" line could be something like
To: "John Smith" <johns@bighotel.co.uk>
the part in quote marks ("…") is the "To:" addressee's friendly name or nickname and the part in angle brackets (<…>) his/her email address.  However this isn't necessarily who that particular instance of the message was addressed to: if you look at the headers the top line is usually
Envelope-to: someuser@someuser.plusnet.com
and that is who that particular instance was sent to.
So filtering really needs to be done on the "Envelope-to:" field if that option is available to you.