As we all know, businesses and internet providers around the world are implementing new and more aggressive spam blocking systems all the time, and this is affecting legitimate e-mail transmission. The question has come up recently regarding how can you avoid having your non-spam e-mails from being blocked. We've had the same question here, so I put together a quick (but not short) list of info items and references that you might find helpful.
Infrastructure - Are you blacklisted?
First, many people use a system that rejects e-mail from known or suspected spammers, and this process is called "blacklisting." It's an inexact science, and there are many different companies that provide black listing services. Here is a pretty good FAQ that talks about common blacklisting issues:
http://dedicated.pacbell.net/faq/FAQs_blacklist.html
Here's a big list of blacklisting sources and informational web sites:
http://dmoz.org/Computers/Internet/Abuse/Spam/Blacklists/
One problem with blacklisting, is that contrary to what you will read in many resources, most systems that use a black list system do not send you a bounce message, so you are not told that you've been blacklisted. Some blacklisting services will attempt to contact your system admin to let them know they've been blacklisted, but not always.
One quick way to determine if you have been blacklisted is to use one of these sites - you put your mail server's IP address in and they check a number of the more common black lists. They aren't the most user friendly system, but are pretty straight forward:
http://openrbl.org/
http://relays.osirusoft.com/cgi-bin/rbcheck.cgi
Infrastructure - Open Relay
Your mail may be rejected, or you can be black listed if your mail server allows anonymous people to send mail from your system. This is how most Spam originates, so if your server allows this - Open Relay - it's only a matter of time before you will be blacklisted. You can also be blacklisted if you have another server on your network that allows open relaying - many people configure Windows 2000 server with the SMTP service running, and forget to secure or block access to it. It is possible that your entire network address can be blocked, so all open relaying should be disabled.
More information on Open Relaying can be found in this FAQ:
http://www.ordb.org/faq/
Infrastructure - Reverse DNS lookup
Many e-mail systems are starting to do what is called a reverse lookup on all e-mails that arrive, and reject those that come from systems without a reverse lookup. What this means is that the system will look at the IP address that the e-mail comes from, and then lookup that IP address and verify that it matches the domain the e-mail claims to be from - if they don't match (and Spam usually doesn't), the e-mails are rejected. The problem with this approach is that many legitimate e-mail systems simply don't have a reverse DNS entry, and this is particularly common when the e-mail is not coming from a corporate e-mail server, but an individual user who has their own domain name, but is using a major e-mail provider such as AT&T, Comcast, or another high speed internet provider.
For companies, you need to make sure you have the reverse DNS lookup configured. You can determine if this is the case by using this web page and entering your server's IP address in the Reverse DNS lookup field and clicking the RevDNS button:
http://www.dnsstuff.com/
E-mails - Content
Most spam filters use the content (or lack thereof) to help determine if an e-mail is spam, and work based on a (growing) number of assumptions. One of the most important things to do is to avoid key spam words or phrases. Rather than run the risk of having this e-mail blocked for including them, I'm including a number of web sites that have some great lists of words you should avoid.
Some content lists:
http://www.wilsonweb.com/wmt8/spamfilter_phrases.htmCommon phrases to avoid and alternatives:
http://www.bankrate.com/media_kit/SpamAlternatives.asp
Some other general tips include:
E-mails - Sending
When sending your e-mails, it is better to use a broadcast/e-mail merge option where each person gets an e-mail individually. Putting many addresses in the To:, CC:, or even BCC: fields can flag your e-mail as bulk mail and increase the chances it will be blocked.
E-mails - Testing
If you're going to do a large broadcast, send it to people you know have spam filters and ask them to let you know if they received it, and if it was flagged as Spam. Also, there is a free spam checking system you can try - send your e-mail to sales-spamcheck@sitesell.net and put the word TEST as the first word in the subject line, but otherwise leave the message as is. (If you don't include TEST your mail will simply be deleted). You will get a response that analyzes the e-mail and tells you exactly what components of the e-mail could be considered Spam and roughly how likely it is that it will be blocked. Note: You have to scroll past a bunch of sales pitch to get to the analysis.
Resources:
This web page is intended for marketers in particular, and has a lot of good information and references:
http://www.wilsonweb.com/wmt8/spamfilter_avoidance.htm
"That's Not Spam, That's My Newsletter!"
http://www.frugalmarketing.com/dtb/notspamnews.shtml
"The Golden Rule of E-mail Marketing"
http://www.uniterra.com/faq/Clasificacion.asp?ID_1=7&ID_2=25