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Sorry if this is incredibly basic, but I'm not very familiar with how SMTP works.

I have a mail server (running Postfix with Dovecot) with an SSL cert from a well-known/accepted CA installed and configured properly. There are 2 clients who connect to this server from different locations - each configured to use SMTP + TLS when connecting to the server to send, and POP3 + SSL when connecting to receive. Is it reasonable to consider communications to be secure (at least as secure as SSL can be) between those 2 email clients?

Clearly if the server or either client was compromised, their communications would also be compromised. What potential security holes could there be?

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up vote 4 down vote accepted

Generally yes. Assuming that 'configured properly' applies of course (I'm not going to get into the subtleties of configuring a server securely).

Also, you rightly focused the question to those 2 parties, and correctly noted the fact that other aspects might cause security issues / information leakage (e.g. if the client or server are somehow exposed or compromised). If the message was sent by one of the parties to an external email address on a different server, then the same level of protection is not likely to apply. (This concern might also apply if one or both parties set a forwarder or auto-reply)

However, one major point that is missing from your question, is how the email later gets delivered to the recipients. SMTP handles sending emails. A different protocol is usually used for picking up email from the mailbox. Typically POP3 or IMAP4, but there are other possibilities of course (webmail, Unix mailboxes etc). Is this part of the communication also configured properly and secured in the same way?

In addition, confidentiality of the communication may not be the only concern. In some cases, the parties want to validate the origin of the message, and that the contents hasn't been tampered with. SSL/TLS on the SMTP channel aids in protecting against it, but it might not be sufficient in certain situations. Are those two parties trustworthy? do they trust each other?? I guess there are many hypothetical questions and directions to look at this, but on the pure technical side, of whether the communication layer between both clients and the server are protected from eavesdropping, I think you've made a fair assessment.

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Added details about how the messages are retrieved. –  Farray Feb 6 '12 at 22:12
    
Thanks for clarifying. I think my answer still applies and generally speaking, the transport layer exchanging the messages between the parties is therefore secure. –  Yoav Aner Feb 6 '12 at 22:14
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Confidentiality. As far as I can see, you'll probably be fine in practice against eavesdropping, though the guarantees aren't ironclad. For instance, suppose the SMTP server is down. The client might look up a secondary MX and deliver the email to the secondary MX server, which might then go over an unencrypted link. Also, the approach you have sketched is pretty dependent upon the specific configuration of all three machines, so it may be a little fragile in practice.

Authenticity. There are no guarantees about the authenticity of the emails. An attacker can easily inject a forged email with a spoofed From: header. The approach you sketch does nothing to make that any harder.

Overall. If your main concern is confidentiality and passive eavesdropping, it's probably good enough for most purposes. It is probably not as robust or solid as end-to-end encryption (though it might be easier to configure or easier to use in practice).

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The security of the email would largely depends on the security of the physical server storage. Since your solution does nothing to encrypt the email while it is stored on the server, then it does little for the overall security as most breaches take place on the server side (probably close to 99%), and not while email is in transit.

For any meaningful security you additionally need to use encryption for email storage. The best would be to have each message encrypted on the client side with its own set of keys, with private key never leaving sender's computer in plain form. There are several solutions that provide such strong security. I would look into pgp email solutions or http://cryptoheaven.com

However, the solution with only SSL encryption for the client connections does provide appearance of security for an average consumer, but not much real protection indeed.

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I appreciate the answer, but I did acknowledge in the question that a compromised server = compromised communications. –  Farray Feb 7 '12 at 23:54
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