I've got an application that sends out notification emails to users of the application (this is not spam; the information in these emails is solicited and useful, and is also a feature turned off by default and must be enabled by the user). The app is still in beta, and one of our testers reports that the notification emails are going to his junk mail folder in Outlook 2003. This is the only reported case of this, but I asked him to send me the email headers from the message, and I noticed that there is a header there labeled "X-CAA-SPAM" with a value of 00000 .

I'm a programmer, so I'm fairly green in the world of successful automated emails - does anyone know if this header is the culprit? If not, any suggestions?


The X-CAA-SPAM header might be unrelated to Outlook flagging the emails as spam.

This page seems to suggest that the X-CAA-SPAM header is being added by Comcast, and would most likely be ignored by Outlook, AFAICT.

Outlook's spam filter might have been trained by the user to reject similar email; providing the usual advice to your users when they enable your email notifications (including "Please add xyz@example.com to your contact list" and "Check your spam box if you don't receive an email within 30 minutes") might be the easiest way to solve it.

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    X- headers are used by many antispam systems to mark their calculations for that email. Including this info as a header makes it easier to troubleshoot how individual messages were caught or missed after the fact. In this case, it appears that the antispam system didn't find the message suspicious enough to block it but obviously Outlook did. If you want more detailed analysis, you might post the full headers of the message so we can look for other stuff that might be suspicious to Outlook since this X-header doesn't appear to be related. – icky3000 May 18 '10 at 6:10

It might be a type of anti spam "hashcash" which is used to increase the amount of computational resources needed to send a email making it theoretically more expensive to send bulk email, https://en.m.wikipedia.org/wiki/Hashcash

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