While this is true, any protocol or system can be hacked.

I understand what you are trying to say.  I'm merely proffering the
additional guideline:  Any required human activity must be enforced with
laws.  Otherwise people simply won't do it.

Spamming is a lot like shoplifting.  People do it because it's easy.
While any individual act of shoplifting is small and rarely involves
serious crime like murder, collectively it hurts a lot, and merchants
complain.

The online community needs to complain better to get attention of their
elected representatives.

IMHO, spam is not a protocol problem.  There are, to be sure, some
issues with SMTP's design, but you might be surprised to learn that
SMTP can now and does often run over TLS, so encryption is there,
and with systems like domainkeys, servers' identities can be verified
as well.

But the real problem is not encryption.  The real problem is the
set of requirements for e-mail.  E-mail has the following features,
none of which does anyone want to give up:

1.  Delivery costs are unmetered.
2.  I can receive an e-mail from someone who I have not previously
    contacted electronically.

#1 is a requirement for any sort of mass mailing.  And it is not
only for-profit corporations that do a lot of bulk mailing; there
are plenty of internet communities that do bulk mailing, and
IMHO should be able to continue to do so.  For example, mailing
lists, or web sites that send messages out when people click the
"forgot my password" link.

#2 is a requirement if you ever want to meet someone in the real
world, give them your e-mail address, and be able to receive a
message they give you.  You might want to do that if you put your
e-mail address on a business card, or write it on a piece of paper
and hand it to some cute MOTAS you meet somewhere, or if you want
to go to a concert and write it on the artist's sign-up sheet for
info on when their next tour is.

When you put requirement #1 and #2 together, the combination
implies spam.

I hate spam, but not enough to give up either #1 or #2.  E-mail
is valuable partly because of the free flow of information that
is possible.

  - Logan

P.S.:  Note that you could actually achieve requirements #1 and #2
       by requiring everyone to have a verifiable identity.  But
       then what organization controls who can and cannot get an
       identity and vouches for those who do have one?  It would
       have to be an organization that can meet the needs of (and
       safeguard the interests of) everyone on the planet.  A web
       of trust could be employed, but now you're talking about a
       fairly high-maintenance solution, and most people don't want
       to worry with signing others' keys and stuff like that.