Difficult to learn
Subtle differences in interpretation
One of the problems with the email and the protocols used to transfer it (SMTP) is that they were designed long ago when the Internet was a much friendlier place. When SMTP was designed it was assumed that other hosts on the Internet could be trusted. This is particularly visible in the configuration of relays where the sender doesn’t have to be identified. A mail relay will accept mail from any server regardless of where the mail appears to be coming from.
To attempt to rectify this SPF was created. To setup SPF you add either a TXT or an SPF record to the DNS zone you will be sending from. This record defines which servers are allowed to send mail that is coming from that domain. So on my domain danielhall.me I could publish an SPF record that says only my mail server is allowed to send mail that ends in @danielhall.me. Any mailservers receiving mail that is from my domain but not coming from an address listed in my SPF record can see that the mail is likely forged and throw it away. SPF works well in most situations but fails at a very common use case. If someone I send mail to tries to forward it to another address using an automatic process (no clicking forward in their client) then the mail will appear to come from my domain when it gets to the user it was forwarded to, however it will have came from the original recipients mailserver.
DKIM solves this problem by giving each sending mailserver a cryptographic key pair. The public keys is then published in a DNS record in that zone and stores the private key somewhere safe on the server. The server then proceeds to sign the headers (especially the From: header) and the body of all outgoing emails. This signature is then attached to the email as an extra header. When the receiving server get the email it gets the signature and uses that along with the list of signed headers to verify the signature against the public key of the signing domain. This means as long as the mail has passed through an authorised mailserver at any point it will be considered valid.
Setting up a DKIM is relatively simple process. You will need access to the zone records for your domain and access to the configuration of all the mailservers which all mail originating at your domain passes through. You also need to be aware that signing mail makes it slightly more processor intensive to send an email. If you send a large amount of email this difference could be quite significant. If you’re using sendmail you may be able to alleviate it by switching to a less resource hungry MTA like Exim. You should also note that in some configurations DKIM can not be setup. For example if you use masquerading in sendmail DKIM will always fail as sendmail will modify the from header after signing.
Ultimately DKIM is a good move for the internet community at large, especially when combined with SPF. DKIM mail is assured to come from the sender and can be cryptographically proven so. While it does take more take a little more effort to setup and maintain it assures mail from your domain is secure and can be assured to have come from you or your company. Ultimately DKIM can protect your company against phishing attempts and boost your spam scores.
Random thought: What would Email look like if it were designed today?