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This is largely a question of how MTAs and different components communicate.

I'm curious that if I send an email using SMTP over SSL to my MTA, can it's next jump to the next MTA or MDA be insecure?

I'm guessing there's no guarantee after it's out of my communication channel, just want to make sure.

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I don't think so. My educated guess comes largely from the fact that the last link -- from the recipient's MTA to their inbox -- doesn't have to be encrypted unless they request it to be. (I know you are talking about MTAs and not clients, but any potential chain can be broken there and is outside of your control.) – logicalscope Feb 23 '12 at 19:29
While the next jump will probably not be over SSL, it's still a good idea for you to use it, anyway. If you want to ensure confidentiality of the message, end-to-end, you should look into email encryption products, PGP or S/MIME. But still continue to use SSL... – AviD Feb 23 '12 at 20:45
This is exactly why S/MIME and PGP exist. – tylerl Feb 24 '12 at 6:53
Of course it won't. After it reaches the person, what will happen to the email is unknown, it could be viewed in using an insecure connection. Even an encrypted email if viwed using an insecure connection, once decrypted ( i.e. placed into a system's memory ), means it also will be likely insecure. Your connection might be secured, doesn't mean the data is encrypted, anyone with the capability to interept the data, can view said data. – Ramhound Feb 24 '12 at 18:53
up vote 8 down vote accepted

You can't be sure that SSL will be used all the way, because there's no specification requiring that. When the email was designed, long time ago, no one bothered about encrypting the connection between the sender and the receiver, including any point that transmitted the email. So, to comply with email specifications, no one needs to encrypt it using SSL, and you can't assume it will be encrypted.

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True, but I would go one step further: you can assume that it will not be encrypted. Too many other components on the way, and the default is to not use SSL. – AviD Feb 23 '12 at 20:43

A simple thought-experiment can answer this for you.

Imagine that I run my own mail server for my domain and I don't implement SSL on it. No matter what protocol you use to talk to your mail server, it will have to use plain SMTP to talk to mine.

Since there exists a case where it is impossible for the next hop to use SSL, your suspicions are confirmed.

SSL for SMTP communications was designed to protect the password you use to authenticate to the mail server, not the content of the email.

Taking this a little further, since SSL is not a required part of the email system, even when there is a choice between using SSL and not using it, MTAs will generally choose to use plain SMTP because it's simpler and cheaper.

The final hop (from POP or IMAP server to client) can also be encrypted or not and that is also completely out of your control and completely independent of whether you used SSL to talk to your MTA.

As AviD said in the comments, use PGP, GPG or S/MIME to protect the contents of the email and use SSL to protect the password you use to authenticate to your mail server.

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What if you use SSL to send directly to a SMTP server which you know supports SSL (e.g. Gmail)? Then wouldn't there be no more hopping? – Pacerier Dec 12 '14 at 9:01

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