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We're having complaints from user that emails are showing up in their spam folders. I've checked some of the headers but I can't get a straight answer as to what to check for and what they really mean. Is there a tool I can use to check our emails and possible fix them?

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Can you clarify: this is emails coming in to your system? and, what are you using for spam filtering? –  Ward May 10 '11 at 2:04
    
What are you using as a spam filer? –  DanBig May 10 '11 at 2:22
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If you can post (sanitized) headers we can probably help you out a bit better. –  Zypher May 10 '11 at 3:18

2 Answers 2

It's probably not your fault - and there's only a little you can sensibly do about it.

It would have been helpful if you'd provided an example

It's the service provider of the recipient who is deciding to classify the message as spam. But they will not usually reveal information about why your message has been classified as spam because:

  1. They don't want the spammers to know how to bypass their filtering
  2. They may not actually know - e.g. if they use Bayesian filtering

There is stuff you can do so you appear less like a spammer:

  1. publish SPF records for your domain (and make sure you only publish specific, named hosts)
  2. apply rate throttling to outgoing mail to avoid flooding carriers with mailshots
  3. only send mail from static addresses
  4. check the RBL lists regularly to ensure you've not been blacklisted (see spamassassin for published lists)
  5. run a copy of spamassassin locally and see how it scores your emails
  6. check that your mailing software is not adding additional headers
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When viewing the headers of the emails going to the spam folder, look for a line with "SCL: " in it. The SCL denotes what level (0-9) of spam the message is marked as when it enters the email server. Usually by default, messages with an SCL of 1-3 are sent to the Junk Mail / Spam folder... If you are in a Microsoft Exchange environment, this filtering threshold can be changed through the Exchange Management Console on the server.

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All spam filtering systems add different headers, so SCL isn't the only think you should look for, and is an Exchange only thing. For example spam assassin will as X-SPAM-LEVEL (or something similar) –  Zypher May 10 '11 at 3:17

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