Mail server technology company trimMail provides an excellent news site called "Email Battles" that is devoted to the holy grail of email deliverability in all its forms. They also provide this gratis series of network research tools that will help you figure out several key things. For example, the Mail Server Profiler tool will map and profile all of the mail servers in your domain. You can take the Open Relay Test to see if your mail servers are at risk of being used by others for nefarious deeds. And you can check to see that your DNS records are configured correctly, which is the first step in successful email delivery.
This company offers list suppression management tools that centralize unsubscribe requests from multiple email lists all in one place, so that you can manage things in one fell swoop. In other words, it offers a global opt-out process across your company. So, for example, if your sales force emails a list of prospects and some ask to be unsubscribed from all company emailings, their email addresses will also be removed from your main email list and any segmented lists that they appear on. Another example is if you are emailing on behalf of a third party such as an advertiser or affiliate and people have emailed you directly and requested to be taken off of all sales-related emails from third parties, you can check the advertiser's list against your unsub list and suppress those email addresses from your third-party mailing.
DKIM (DomainKeys Identified Mail) is an open-standard, signature-based email authentication system spearheaded by Yahoo! in collaboration with AOL, Microsoft, IBM, Verisign, Sendmail and other leading companies. DomainKeys essentially gives 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). Under this system, the domain owner generates a public/private key pair to use for digitally "signing" all outgoing messages. The public key is published in your DNS record, and the private key is made available to your DomainKey-enabled outbound email servers. When an email is sent, these keys generate a digital signature that is appended to the email header. On the receiving end, all information is verified to authenticate the email. DKIM does not eliminate or compete with other email authentication technologies such as SPF or Sender ID Framework; it is a sophisticated secondary level of defense. This site will tell you how to start the process of getting your DKIM keys.