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Let's say I have a forward for my domain.com:

email@domain.com -> my_personal@email.com

Now that there is this record, is my personal email known to the public?

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This is not a MX forward. There is nothing like a "MX forwarder". You just forward your mail. –  mailq Nov 12 '11 at 23:07
    
But it's an MX record, right? –  m33lky Nov 12 '11 at 23:11
    
No, it is not. You should get your wordings sorted. –  mailq Nov 12 '11 at 23:21
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3 Answers

up vote 2 down vote accepted

It depends on the type of forwarder. If it's a naive forwarder, then if there's a delivery error with the forwarded mail, the error will go to the original sender, thus leaking the destination address of the forwarded email. If it's a remailer, then no.

Think about it this way: What do you think should happen if the final destination of your email cannot accept the email for some reason (server down, mailbox full, and so on)? And if an error is sent back to the original sender, who do you think that error will be from?

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Many registrars offer this feature, so I'm wondering how they do it. –  m33lky Nov 13 '11 at 0:27
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You can test by forwarding to a non-existent address and then sending an email. Look very closely at the headers in the error reply you get. –  David Schwartz Nov 13 '11 at 0:46
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The intermediate forwarding server may report the destination via the EXPN command as well; though this command is typically disabled in modern e-mail servers. –  Chris S Nov 13 '11 at 1:33
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I'm not quite understanding what you're asking, but each email server that handles the email will prepend a Received: line to the email headers that contains information that can be used to identify that server, although that information might be completely abstract outside of that local system itself.

See section 3.8.2 here:

http://www.ietf.org/rfc/rfc2821.txt

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RFC 5321 section 3.7.2: "(...) a gateway MUST prepend a Received: line (...)" –  mailq Nov 12 '11 at 22:26
    
Oops, I quoted the earlier RFC 2821, although it states the same requirement. –  joeqwerty Nov 12 '11 at 22:31
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No. If you forward mails then the destination email address is never known to the public. Unless you answer the forwarded mail. Then there is a chance that the sender's address is revealed.

Anyway I can't see an advantage in "security". It doesn't matter which email address is the final recipient. As long as there is a mail address of domain.com or email.com known, it will be a target for Spam. So you can hide email.com by exposing domain.com and have won nothing.

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It's not about spam, but more like privacy. You know how for example people like to protect their whois and not provide identifiable information. –  m33lky Nov 13 '11 at 0:25
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