This article was originally published as part of the Pobox blog. Pobox was acquired by Fastmail in 2015.

False positives and false negatives are terms that originally came to us from medical tests. When you perform a pregnancy test, bloodwork, or a scan for a disease, there is the result, and then there is what's actually true.

  • When you are pregnant, but the test says you're not, that's a false negative.
  • When you're not pregnant, but the test says you are, that's a false positive.

The same thing can happen with spam filters. When a message that you actually want to receive is rejected, that's a false positive. The filters saw something that was not spam, but thought it was. False positives are a very serious problem, because you've lost mail that you need!

The easiest way to prevent false positives is to make a complete trusted sender list. That's why we try to make it as easy as possible for you to add trusted senders. You can add them yourself, but we also automatically create them for you when you send mail through Pobox. We also try to learn from our mistakes; when you release a message from the Spam section, you can add a trusted sender automatically.

When you receive a message, but it's spam, that's a false negative. False negatives are still annoying, but you usually have to make a trade-off -- the more aggressive you make your spam filters, the more likely you are to get a false positive, a missing message that you wanted to receive. So, most people accept some small amount of spam in their lives, to make sure they don't miss any of the messages they want to receive.

If you don't agree, we have lots of options for aggressive spam fighters! You can set your spam filters to block mail from countries where you don't know anyone. You can turn our filters up to super aggressive, and bounce messages that you don't want. You can even block everything that comes from someone who isn't one of your trusted senders!