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Can ISP block selected e-mails?

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#1 Cammacci


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Posted 21 October 2009 - 12:42 AM

Hi. I think i'm this is the right place to ask this question.

I'm not interested in legal aspect, just technical. I know it's not right. But there are still governments which practice censorship.

I'm not asking about spam filters. Those can be applied by mail service providers, which is different from ISP. Say my mail SP is Yahoo Mail, my ISP is Comcast for example. Comcast is not controlling Yahoo severs. But can control traffic from Yahoo server to me. Right?

The situation: ISP blocked a website. Now most likely it will try to filter subscribed distribution mail from this site.

Can an ISP filter my emails using information in the headers? The e-mail which I access through browser (not an e-mail client)

If yes, Can this be avoided?

Apparently I'll see the subject line, but will I'll be able to receive the body of the mail? The text? What if there is no text just attachment, say zip file?

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#2 Andrew


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Posted 21 October 2009 - 03:06 AM

In theory, any and all unencrypted data can be seen by anyone between you and the point of origin. Your ISP may, or may not, decide to block data from crossing it's network based on the headers, the protocol, the source, the destination, anything.

Most ISPs outside of the "internet black holes" don't do it, with some notable exceptions1, 2.

The information in e-mail headers is often used to catch spam. It would be trivial to filter for other things. The only real way to bypass that kind of filtering is to switch ISPs. Web-based e-mail would be a bit different, but the basics would be pretty much the same.

If that's what's going on with your ISP and you decide to switch, be sure to tell them why. Let them know that it's not OK to censor the internet.

#3 Cammacci

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Posted 21 October 2009 - 09:55 AM

The reason I'm asking I had experience. In Uzbekistan I tried to open some of my messages but I was getting message from the ISP about an error. Those were plain text, but contained censored words like "Andijan" "massacre" "government". I could access the site, could log in and see the list of e-mails, but couldn't open some. While others were perfectly accessible. That was web-based service - yahoo mail.

And there's only one state controlled ISP just it has many sub-providers with different names. I think they use Chinese channel

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