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If I understand correctly, communication between SMTP servers may be encrypted but is not required. Because it is optional, does it really have benefits? When server A connects to server B, both of which supports SSL/TLS, can't a MITM just jump in and say server B doesn't support TLS?

Edit: I'm referring to the SMTP connection say, between Gmail's SMTP server and my SMTP server, when sending an email from Gmail to @mydomain.com. Is there a purpose for Gmail to use SSL/TLS when communicating with my SMTP server? From my understanding, there isn't anything preventing a MITM attack there.

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1) the big gain is encryption between the user and his SMTP server 2) It blocks passive listeners, such as the rumored NSA listening boxes at big ISPs. –  CodesInChaos Feb 24 '13 at 14:25

3 Answers 3

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Opportunistic SSL will deter passive attackers who just spy on the line. It also slightly raises the bar for active attackers: in order to be able to alter the transmitted data (e.g. to inject a forged email in the stream), the attacker now must make a full MitM with fake certificate and decrypt/reencrypt the whole connection.

But, really, that's for public relations. Big mail relay hosts like Gmail use SSL/TLS when they can mostly to show that they are responsible and made an effort towards "more security". Ultimately, this is not a bad move: if the fashion takes hold and turns into tradition, then we may hope that at one time SSL/TLS will become mandatory. If the SMTP servers then decide to use it properly (i.e. with adequate validation of the peer certificate, complete with revocation checks), then overall security will have increased.

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SMTP is inherently a relayed protocol, with any relay not belonging to the source or target domain being a man-in-the-middle. SMTP would approach confidentiality offered by HTTPS if a SMTP server controlled by your organisation were to look up a mail server from the MX DNS records of the target domain and then only transmit the email via TLS to that server.

Obviously, this would exclude all webmail - where the Javascript client delivers the mail via HTTPS to the webmail provider's HTTPS server, which then decrypts it and then stores/transfers it in clear-text to its SMTP servers and then may or may not send it over TLS to the target domain's SMTP server.

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It's up the client to decide whether it demands encryption. If the client is happy accepting 'no' for an answer then it offers less security. However the client should know (via the user settings) whether the server accepts TLS or not. If the server says 'I don't know what you're talking about' one day, your client should be configured to reject it. If your client doesn't offer you that functionality, I'd question the client.

Basically "communication between SMTP servers may be encrypted but is not required." is a flawed assumption. My client refuses to connect to my SMTP server unless TLS negotiates successfully as expected.

For clarity: Using TLS without demanding it does offer some security against certain types of attack. It's just vulnerable to MITM attacks. For some purposes that's sufficient, although generally speaking it's not what you're after.

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Thanks for the answer. I was actually referring to the connection between Gmail, and say, a third party SMTP server. –  Tech163 Feb 24 '13 at 14:37

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