A guide on email newsletter design wouldn't be complete without a section on spam filters. You have to design your emails a certain way, if you want your messages to make it past them. But you can't go overboard, either. Spam filters can smell fear. If you try too hard, they know it, and they attack. So just be cool, act normal, and don't make any obvious mistakes.

How anti-spam systems work

Before getting into the nitty-gritty design tips, you need to know how all the various anti-spam mechanisms work out there. Once you get an overall understanding of how they work, designing around them will be much easier.

Bayesian Filtering

This is one of the most important ones to learn about, since it's installed in so many email applications these days (like Outlook, Mozilla Thunderbird, and Apple Mail). Bayesian filters work by watching users classify email as "junk" (such as when they click a "this is spam" button). It reads the junk mail, compares it to other emails you called "junk," and looks for common traits in the subject line, the content, the hyperlinks, the sender, etc. Over time, Bayesian filters learn to scan for those "traits" in every email message you receive. Every time they find something that looks "spammy," they assign a "score" to it. For instance, using "Click here!" might get you 0.7 points. Using bright red fonts might get you 2 points. Including the word, "mortgage" might get you 1 point, but using the word, "Viagra" will get you 5 points. Once an email exceeds some threshold (set by the user), the email is classified as spam, and thrown into the junk folder. It's amazing how many different things Bayesian filters look for.

euro.message LIVE also comes with a spam filter checker. It'll tell you exactly what your email's spam score is, and what you need to change if you want to improve your chances with spam filters.

Black Lists

A while back, some server admins got really, really, angry at all the junk mail they received. So they started to track the IP addresses of the servers that sent them spam, and put them on "blacklists." Anytime they received email from a server on their blacklist, it was deleted immediately. It worked pretty well. So they started to share their lists with other server admins. And other admins started to add on to those blacklists. Pretty soon, the blacklists got very, very big. Large ISPs started to sync up with them. If you send email that's very "spammy" you could end up on one of these blacklists (whether the recipients are opt-in or not). Once you make it on to a blacklist, good luck getting off. Geeks are notoriously difficult to negotiate with (we know, because we're geeks). This is why you need to make sure you only send to recipients who gave you verifiable permission to email them, and you need to have proof that each one of them opted in to your list (such as through a double opt-in system). Want to know the easiest way to get blacklisted? Let your company's sales team blast out an email newsletter to a list of "prospects" that they collected from conferences, and that they scraped off of websites. You'll get blacklisted, guaranteed.

Email Firewalls

Managing email servers (and incoming spam, viruses, and phishing attacks) can be a lot of work. So large corporations usually install "email firewalls". Think of them as "spam filters on top of spam filters." They're big, heavy-duty gatekeepers, and they're not friendly at all.

They often use a combination of Bayesian-style/adaptive filters, community reporting, blackhole lists, and a little bit of proprietary "magic pixie dust" to keep spam out of the company. Most of the time, when your email's not getting through to a larger company, it's their firewall. You can think of these firewalls as kind of Xenophobic and paranoid. They're all twitchy, and tend to ask questions like, "Okay, is this sender new to me? Why is he sending copies of the same, exact email, to a bunch of people in our company? Can I really trust this sender?" Spam firewalls are usually only a problem when you first start sending campaigns to a big client or something. You'll experience some deliverability issues in the beginning, because you're "new." They'll eventually "learn" to let you through. To expedite things, you may have to ask the IT people in charge of the firewall to "white list" your IP Addresses (or the IP address of your email service provider).

Challenge/Response Filters.

These are more common among "at-home" recipients (because they're too intrusive to use at work). When you send email to someone with a challenge/response filter, here's what happens. If you are not already in that person's "buddy list" or "address book," then you're considered a stranger to him. And if you're a stranger, you could be a spammer. So their challenge/response filter sends you an automatic reply with a question that you have to answer, or some link you have to click (this is to prove you're a human, and not a spambot). The thing to remember here is that you have to be "white listed" if you want your emails to get through. So when people fill out your opt-in forms on your website, ask them to "please add our email address to your address book..." Use your opt-in process as a way to "set expectations" and get "whitelisted" up front. Whenever you send a newsletter, make sure the "reply-to" address that you use is valid, and that a human checks it after each campaign.

Tactics for Avoiding Spam Filters