Is it more blessed to send than retrieve emails? Mandatory-TLS transmission vs IMAP over SSL retrieval

Question

A client (another company) wants us to send them info via email regularly (hundreds of emails a day). They requested emails because each email will create a separate ticket in their task management system.

The emails contain private information. We can set up Mandatory TLS (a.k.a. "policy-enforced TLS") on our mail servers to make sure the emails are encrypted in-transit.

My question is: would it be better, instead, to set up a mailbox on our server and have the client retrieve the messages from this mailbox via IMAP over SSL? My first thought was that this would be more secure, since the mail would not be transmitted between unknown intermediate mail servers, but then I remembered that TLS is the successor to SSL, which I presume means stronger encryption.

So which is more secure: sending the email to the client's mail server, via policy-enforced TLS, or having the client retrieve mail from a mailbox on our mailserver via IMAP over SSL? (Our server doesn't seem to support IMAP over TLS; not sure if that is a thing.)

For the purposes of this question, assume that we have secure mail transmission within each respective company: I'm just asking about securing the mail transmission between the two companies. You can also assume secure mail storage on the two respective mail servers.

There's a similar question about Secure third-party inboxes, but such inboxes would be accessed via a browser, presumably with a TLS connection, not via IMAP over SSL, so I don't think it applies here. The client will retrieve messages programmatically.

My instinct is that having the client's task management system retrieve the messages via IMAP over SSL is more secure because it has fewer "parts", so there's less things that can go wrong. What isn't clear to me is how much benefit there is to using TLS instead of SSL.

Answer 1

TLS in SMTP is hop by hop encryption, i.e. the mails are only encrypted between each mail server in transit but each mail server involved has full access to the decrypted mail. Otherwise these MTA would not be able to add Received or DKIM-Signature headers or similar to the mail.

Apart from that, the sender can only control if TLS is used to transmit the mail to the next hop and the recipient can control that the mail is only received by TLS from the last hop but both have no control how the mail gets transmitted between any other hops. And, TLS does not help against DNS spoofing or misconfiguration when the attacker can control which MX is involved in delivery.

IMAP with TLS instead is end-to-end encrypted. Insofar the IMAP solution would be more secure against eavesdropping by design. You could only reach a similar security with SMTP if you have control of all the hops involved. Depending on your outgoing mail server this might be done by configuring a fixed and direct route to a specific mail server of the recipient instead of relying on the common mechanisms (MX lookup) to automatically find the route.

but then I remembered that TLS is the successor to SSL, ...

TLS and SSL are just different names the same protocol family and are often used interchangeably or are just called SSL/TLS unless a specific protocol version is meant. In fact, TLS 1.0 is practically SSL 3.1 and the amount of differences between difference between SSL 3.0 and TLS 1.0 might even be smaller than between TLS 1.2 and TLS 1.3.

... which I presume means stronger encryption.

Better encryption does not help at all if the attacker is not trying to sniff the traffic during transit but at the mail servers involved which is possible with the hop-by-hop encryption in SMTP but not with the end-to-end encryption in IMAP.

Answer 2

I would suggest its more secure for the party that hold the sensitive data to push it, instead of having the other company pull it.

The reason is that, imagine the IMAP password leaks, or a untrustworthy server administrator gets hold of these details and pulls data from IMAP.

By pushing the data instead, you ensure the data end up where it should. And as long as you push to the correct SMTP server, theres no intermediate SMTP servers in-between.

You can easily use a policy that also forces a specific server-IP for a specific email domain, in addition to the forced TLS.

Regardless of if you use IMAP or SMTP to transfer, you can in regards to SSL/TLS, configure them with equal security, so encryption-wise theres no difference.