Spam filters are getting better every year. They have to so they can keep up with the ever-increasing flood of spam. But no matter how good the filters get, some spam will always leak through. More worrying, some fraction of good messages will be inappropriately tagged as spam and lost. And depending on how your respective spam filters are set, your reader may never even know that the message was attempted nor you that the message was rejected.

A while back, we wrote a tip about "how not to look like a phish". I’ve wanted to write the companion article about not accidentally tripping the spam filters for several years now. I resisted because the rapid change in spammer tactics makes any list obsolete even before it hits the page. It will also never be a definitive list – the anti-spam vendors are justifiably worried about giving the spammers a roadmap showing how to bypass their filters. Nevertheless, there are some general rules worth discussing.

  • Your subject line is important. A blank subject line (or, worse, a subject line that is ambiguous and generic like "Hi" or "I love you") will almost certainly get your message tagged as spam. A good subject line is also a courtesy to your readers, helping them to more quickly prioritize their inboxes and give your email the attention it deserves.
  • Mailing to lots of people at once will increase the odds of being tagged as spam. (This is a problem for the publishers of legitimate email newsletters with large distribution lists like, say, these tips.)
  • Use a company-issued email address. Sending from a free email account like or gmail will increase the odds of getting tagged.
  • Avoid common spam words like "cheap" and the V- word (rhymes with the famous waterfall). That sometimes means completely avoiding certain topics (which can be quite difficult, especially in a newsletter like this one where we are discussing spammer tactics) but more often means avoiding flowery, inflammatory or overly-promotional language. In particular, avoid all caps and multiple exclamation marks.
  • Avoid images, fancy graphics and html code in your email. Hackers and spammers hide things in those glossy "enhancements". The simpler your message, the more likely it is to get through unmolested.
  • SPELL-CHECK! Spammers are getting much better at the use of grammatically correct English but bad spelling is still a surprisingly good filter for spam.
  • If you are sending a newsletter, always include your real contact information and a working set of “unsubscribe” instructions at the bottom of the message. This won’t actually help you get past the spam filters – too many spammers just include fraudulent unsubscribe options in their messages – but it is the law.
  • Try to keep your message under two megabytes including embedded pictures and attachments. This isn’t strictly a spam-filtering rule but many mail servers use a 2 meg/message limit to keep any one message from tying up the lines.

Finally, if you don’t get an answer in a reasonable amount of time, follow up on your message. No matter what you do or how good the filters get, some false positives will always exists. The person might be ignoring you but it’s more likely that they never got the message.

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