Different standards

Public Key Association

Public Key Association(PKA) is a scheme that base itself out of RFC4398: Storing Certificates in the Domain Name System (DNS) which got published in March 2006 and obsoletes RFC 2538.

The abstract of the RFC says as follows:
Cryptographic public keys are frequently published, and their authenticity is demonstrated by certificates. A CERT resource record (RR) is defined so that such certificates and related certificate revocation lists can be stored in the Domain Name System (DNS).

The mentioning of PKA implementation shows up in the release notes of GNU Privacy Guard

The GnuPG release announcement for version 1.4.3 contained the following:
Implemented Public Key Association (PKA) signature verification.
This uses special DNS records and notation data to associate a
mail address with an OpenPGP key to prove that mail coming from
that address is legitimate without the need for a full trust
path to the signing key.

What is GNU Privacy Guard

GnuPG is the GNU project's complete and free implementation of the OpenPGP standard as defined by RFC2440 . GnuPG allows to encrypt and sign your data and communcation, features a versatile key managment system as well as access modules for all kind of public key directories. GnuPG, also known as GPG, is a command line tool with features for easy integration with other applications.

Sender Policy Framework

Quoting openspf.org for explaining what the sender policy framework does:

Have you ever gotten spam from yourself? I have, and I've been thinking hard about how to stop it! I didn't send it. It came from a spammer. If we could stop spammers from forging mail, we could easily tell spam from ham and block the bad stuff.

SPF makes it easy for a domain, whether it's an ISP, a business, a school or a vanity domain, to say, "I only send mail from these machines. If any other machine claims that I'm sending mail from there, they're lying."

When an AOL user sends mail to you, an email server that belongs to AOL connects to an email server that belongs to you. AOL uses SPF to publish the addresses of its email servers. When the message comes in, your email servers can tell if the server on the other end of the connection belongs to AOL or not.

And that's it! SPF aims to prevent spammers from ruining other people's reputations. If they want to send spam, they should at least do it under their own name.

And as a user, SPF can help you sort the good from the bad. Reject mail that fails an SPF check. Use it to help your spam filters make a decision. Have confidence that mail that SAYS it's coming from your bank, your credit card company, or the government really is!

If you do get spam that passed an SPF check, then you know you should hold the sending domain responsible for the message.

The sender policy framework depends on a TXT record in the DNS zone. The DNS entry for KF Webs could look like

kfwebs.net. IN TXT "v=spf1 a mx ~all"

You can read more about the SPF at en.wikipedia.org

DomainKeys

DomainKeys, abbreviated DKIM, is currently being used by amongst others Google in its gmail service and Yahoo. An example of a mail header is:

DomainKey-Signature: a=rsa-sha1; q=dns; c=nofws;
s=beta; d=gmail.com;
h=received:message-id:date:from:to:subject:cc:mime-version:content-type;
b=Ud2KRmZ1JjQ8GeDzW+HGu6QOYo+TF1TWlznsQE48j5cEESiUIhS4+cMLbH0iuSNEBvsS2b (wrapped)
v1TQsCw7lnMkHxDLuccFGc033TReCVYdiAmbQVoEspFYGjl79pYW0+RHyp1AAZ96fTs+4h1S (wrapped)
WeNJ4B3fuHMOAIJPa4k81hS+F9MoE=

antispam.yahoo.com says the following about Domain Keys

DomainKeys: Proving and Protecting Email Sender Identity

Email spoofing - the forging of another person's or company's email address to get users to trust and open a message - is one of the biggest challenges facing both the Internet community and anti-spam technologists today. Without sender authentication, verification, and traceability, email providers can never know for certain if a message is legitimate or forged and will therefore have to continually make educated guesses on behalf of their users on what to deliver, what to block, and what to quarantine, in the pursuit of the best possible user experience.

DomainKeys is a technology proposal that can bring black and white back to this decision process by giving email providers a mechanism for verifying both the domain of each email sender and the integrity of the messages sent (i.e,. that they were not altered during transit). And, once the domain can be verified, it can be compared to the domain used by the sender in the From: field of the message to detect forgeries. If it's a forgery, then it's spam or fraud, and it can be dropped without impact to the user. If it's not a forgery, then the domain is known, and a persistent reputation profile can be established for that sending domain that can be tied into anti-spam policy systems, shared between service providers, and even exposed to the user.

For well-known companies that commonly send transactional email to consumers, such as banks, utilities, and ecommerce services, the benefits of verification are more profound, as it can help them protect their users from "phishing attacks" - the fraudulent solicitation for account information, such as credit card numbers and passwords, by impersonating the domain and email content of a company to which users have entrusted the storage of these data. For these companies, protecting their users from fraud emails translates directly into user protection, user satisfaction, reduced customer care costs, and brand protection.

For consumers, such as Yahoo! Mail users or a grandparent accessing email through a small mid-western ISP, industry support for sender authentication technologies will mean