Today, I saw a question on a distribution list that I participate (Seattle Tech Startups) about a startup being blocked by email servers and not being able to deliver email to its customers.
If email is a key element on your strategy to attract and retain customers, and you are one of the technical persons responsible for that, do read this...
Once upon a time...
When I started Sampa I had a lot of problems with our confirmation email not reaching our customers. It wasn't they were going to the Junk Folder, it wasn't even getting there because spam filters were deleting it. So, through a lot of pain, investigation, phone calls, tips from other startups, we improved our distribution of email considerably.
Here are some of the tips I recommend you to try:
Tip 1: Reverse DNS
Make sure you have Reverse DNS setup correctly. If you don't know what it means, ask something that understand DNS. Basically, if your domain is "sampa.com" and it resolves to "207.115.80.184", make sure that a reverse lookup to that IP address will resolve to your domain name.
Tip 2: Use a single SMTP outgoing server
Always send your emails from the IP address above (from Tip 1). Don't use a separate "email server" unless you also setup a DNS/Reverse DNS entry for that sub-domain (like "mail.sampa.com")
Tip 3: Age Matters
If your domain was created or acquired recently, that matters a lot for Spam filters. Try to purchase your domain months (or a year) before you release your product. Also, avoid using ID Protection on your Domain WHOIS information.
Tip 4: Don't include the obvious spam words
You probably know that including "viagra" on the title of your message is guaranteed to get you to a Junk Folder, but there are many other words you should avoid using on the title or body of your message, like "cialis", "debt reduction", "mortgage", "mesothelioma", "paypal account", etc. Think about all the spam that you received and what keywords they contain.
Tip 5: Use "On Behalf of" when appropriate
Google Groups and Yahoo Groups use that often. Why don't you? If user A is send an email to user B through your site, don't impersonate user "A", but send as "Sampa on behalf of A". Now, this is a bit more complicated SMTP construct (it's not just changing the TO address), so learn how to do that. On .NET it means setting the "Sender" property.
Tip 6: Who are you?
Some companies and spam filters will give you bonus points (as to not end up on the Junk Folder) if you include your physical address and your phone number at the footer of the message.
Tip 7: Let your users out
If you don't include an easy unsubscribe link at the footer of your message you will certainly be penalized by spam filters and by human reviewing your email messages. Plus, you'll get some of your users angry! Which leads me to...
Tip 8: Take care of customers complaints
Promptly reply to request from your customers to be removed from the list. Either remove them once they request it, or give clear and easy instructions on how they can opt out of that category of email. Some users complain to their ISP and if the ISP receives just a few of those, they will block your IP.
Tip 9: Call the ISP
If you were blocked by AOL, Yahoo, MSN, EarthLink, etc., call them. Go to a search engine and find out how to contact them. We've been blocked by many ISPs until we called them.
Tip 10: Don't bounce twice!
Every time you get an email that bounced, for example, because the user misspelled his address, just remove that from the list. Keep sending messages to the same email address that bounces over and over again is a sign of a spammer. Are you one?
Tip 11: Get users to help
The more users that add your domain to the "Safe Domain List" the better. Most services use that criteria to increase your "points".
Tip 12: Sender-ID and more
There are technologies that are likely to increase the number of emails that get through, however, they are new and not widely tested or accepted.
Tip 13: Buy your way in
Have money? GoodMail Systems can get you through.
The tips above are the "obvious" ones ('obvious' meaning if you get banged on the head enough times they become obvious). There are another bunch that you can play with users behavior that can also make a huge difference, but that you'll have to figure out on your own, or hire me to tell you.
Sampa continues to climb on the list despite the fact that Compete proves once more they are out of whack because they show a drop in our reach. Alexa, Google Analytics and our own internal stats have the same growth level, which validates that we are actually growing at a steady and fast pace.
We moved 1 position to #28 in Seattle, just behind BigOven, which, by the way, is getting a lot of press today.
For the first time ever, our Alexa ranking is below 40,000, more specifically at 38,960. That is pretty amazing if you think about there are millions and millions of sites and services on the Internet, and we are on the top 40,000. Actually, we would be on the top 10,000 if Alexa (or Compete) was better at tracking page views because they don't count any of our IFRAME pages, which Sampa relies a lot on.
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I had a lot of problems with our confirmation email not reaching our customers. It wasn't they were going to the Junk Folder, it wasn't even getting there because spam filters were deleting it. So, through a lot of pain, investigation, phone calls, tips from other startups, we improved our distribution of email considerably. 
October, 2007