Here are a couple of links regarding improving email delivery. Email service providers continually are tweaking their approaches to ensure that the emails they're sending are landing in the most INBoxes as possible.

Large email providers don't want to deliver spam, as it sucks bandwidth and irritates users. To help limit the amount of Spam, a number of approaches are used to authenticate a sendor.

Excellent Resources

1) http://www.returnpath.net [3] (corporatist)
2) http://blog.deliverability.com/ [4]
3) http://spamlinks.net/prevent-senders.htm#email [5]
4) http://www.pivotalveracity.com/email-marketing-resources/isp-postmaster-... [6]

Keys to Landing in the InBox

1) Sender Reputation - Most important factor. Long term.
2) The SPF Record - Sender Policy Framework. (DNS, 5mins)
3) Domain Keys - Need provider/admin to handle. Emerging.
4) SenderID - The same as SPF, but checks different stuff.
5) Feedback Loops (FBLs) - These are services maintained by large ISPs that notify senders that a user has marked a message as 'spam.' Relates to reputation. (See http://recognizedexpert.com/marketing/email-marketing/list-of-feedback-l... [7])

Active Campaign has posted a very good primer on the subject at http://www.activecampaign.com/email-marketing/deliverability [8].

Blacklists

http://www.rackaid.com/resources/how-to-remove-your-email-server-ip-from... [9]