If you send email campaigns long enough, you will inevitably run into spam filter issues. We've found that on average, you can expect 10-20% of your emails to just get lost in cyberspace, mostly due to overzealous spam filters. You don't even have to be a spammer to be spam-filtered. Innocent email marketers who send permission-based emails to people who requested them get spam filtered all the time.
Unfortunately, there is no quick fix. The only way to avoid spam filters is to understand how they work.
Generally speaking, spam filters look at a long list of criteria to judge whether or not your email is junk. For example, they might look for spammy phrases like "CLICK HERE!" or "FREE! BUY NOW!". They'll assign points each time they see one of those phrases. Certain criteria get more points than others. Here's a sample of criteria from Spam Assassin, one of the most popular spam filters out there:
If your campaign's total "spam score" exceeds a certain threshold, your email is sent to the junk folder. You're probably going to ask "what's the threshold I need to stay under?" Sorry, but the threshold is different for every server. It's determined by the person who installed the spam filter software. If the person is really sick of spam, they'll set the threshold extremely low. Just about anything will get spam filtered then.
Oh yeah, that list of "spammy" criteria? It's constantly growing and adapting, because spam filters "learn" what junk looks like, every time someone clicks the "This is spam" button in their email program. Spam filters even sync-up with each other online, to share what they've learned. See why there's no magic bullet?
These are the most common mistakes we see new email marketers make, which result in accidental spam filtering.
How can you tell if your campaign ended up in recipients' junk folders? For starters, look at your open rate. If it suddenly dropped from your average, you probably have a spam filter problem. If you're new to email marketing, 20-30% is a rough open rate average.
An abnormally high bounce rate is another indicator. Look through your hard bounces, and read the SMTP replies. Spam filters sometimes leave little clues about why they blocked your campaign (See: How to read your bounceback headers from our knowledge base).
Most established email marketing services (like MailChimp) have been accepted into feedback loops with ISPs like AOL, Netzero, MSN, Hotmail, and more (See: "Getting in the Feedback Loop at Clickz.com"). Whenever a recipient on their network reports an email as spam, an alert is sent to the sending server. MailChimp receives those alerts and stores them under your account, so you'll know how many people reported your campaign as spam (MailChimp will also remove those people from your list automatically, so you don't get reported again).
MailChimp actually comes with a spam checker tool that you can use to thoroughly scan your email campaign for spammy keywords, plus all the other "behind the scenes" stuff that spam filters look for (like image weight, and HTML coding errors). It can save you a ton of time and money by just running one test before you send your campaign. More information about MailChimp's Inbox Inspector Tool...
Perhaps the best piece of advice we can offer to new email marketers is this: Go and look inside your email program's junk folder. Look at all that junk. Really analyze how they designed and coded their emails. Then, don't do what they do.