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You need to have things setup correctly for your e-mail server to operate well on the internet and to ensure that other servers will accept your e-mail. This includes configuration issues as well as registrations at the big networks that have their own processes you have to follow (includes AOL, Time Warner, Yahoo, Microsoft and others).
This is not an exhaustive article on how to run an e-mail server. However, you certainly want to make sure a number of things are setup correctly, especially if you run e-mail lists of any type. Make sure DNS for the domain has a proper e-mail address that is monitored and make sure you have postmaster@, abuse@, security@, and email@example.com pointing to someone since these are considered standard addresses for ISPs.
Make sure you're at least doing these.... Most of these are important, but not dealt with by first-timers. Some, like SPF, are technically optional, but you want to do them anyways.
Make sure your server doesn't isn't an open relay which allows those that shouldn't to send messages through your server (typically without authentication or an improper configuration). You can find many testers on the web, but the best ones I've found are above.
You want to make sure that your IP addresses have reverse DNS setup on them, preferably so that they match the domain used in the server's HELO message, but at least to something that resolves back to the same IP address when looked up by that name. You can check the other DNS settings for your domain using one of several DNS checkers:
Sender Policy Framework
Sender Policy Framework (SPF) is the open-source, DNS-based version of all those for-pay e-mail certification services (remind me why I need to pay you to allow your clients to accept my e-mail -- are you going to pay me to accept yours?). Essentially SPF is a DNS-queriable configuration information that specifies what servers send e-mail for the domain. This allows receiving servers to make sure the message originated from a known good server and/or to inspect it further if not. More info and a configurator at the OpenSPF Project.
Backscatter was a new one on me and I learned more about it from Backscatterer.org. Apparently those "does this from email address actually exist" queries to the domain of the from address can bring a server to its knees, and it's no longer considered best practice -- at least by some. I had to turn it off because of complaints by some clients having problems and found this consideration. Turning off these checks increased dramatically the amount of spam incoming, however, so I probably will need to address that again.
Make sure your IPs aren't on the OpenRBLs (real-time black lists -- of spammers and bad service providers). Investigate and get fixed and corrected if you are. If you've been assigned an IP already on this, ask your ISP for another IP that isn't blacklisted as it's a bit of work to get off. Sign up with SpamCop to receive reports for your IP addresses/ranges as they are one of the first reports and most widely used overall.
This one has been generally abandoned and you don't want to be using it. Why not? Well try this well-written article on why Challenge-Response Anti-Spam Systems are Considered Harmful.
Web Pages / Emails
Don't forget to have web pages on your site with your policies and unsubscribe, subscribe info. Include links to these on every e-mail. Make sure the from address actually exists.
Then there are the big guys, and you have to deal with each of them separately. Keep the IP addresses up-to-date. I'll continue to fill these out as we deal with them again. Might also look at the Word to the Wise Delivery Wiki.
One of the first network providers from pre-Internet days. Now owned by Time Warner, but operated separately to some degree still. At the very least, setup an AOL Email Feedback Loop for your email servers. You can also lookup error messages on the AOL Postmaster Error Page, or reach the AOL Postmaster via email or their postmaster webpage. You can request being whitelisted (if you send more than 100 messages/month to their servers).
(MSN, Hotmail, etc.) These guys were the hardest and most frustrating to work with. I exchanged lots of email with them and still ran into problems. However, use their Smart Network Data Services (SNDS). You will have to register with a Microsoft Live (formerly Passport) account (used for many things like. After logging in, you have to Request Access for IP addresses and once granted (requires confirmation email), then you can view the traffic on their servers. If you change IP addresses, you need to update it at their site.
Yahoo Postmaster suggests using DomainKeys (tech info on DomainKeys). DomainKeys is required to participate in the Yahoo Feedback Loop Program (ISPs can use request an inclusion based on their IPs alone). However, you can also use the Yahoo! Mail Delivery Issues Form if you run into issues. If you run primarily bulk e-mail (assuming solicited of course), then use the Yahoo! Mail Bulk Sender Form instead.