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When an email recipient server (say server-A) is configured to forward to another email addresses (on say server-B), is TLS applied all the way up to server-B, or does it end at server-A, or something else?

Below I'm sharing my rather concrete example to help giving more context for my question, but this is only an example and the essense of my question is above.

I have a forward-only email address my college operates for alumni, and I set it to forward to my gmail.com address. Looks like TLS has been failing for awhile, some emails in my gmail inbox viewed on GMail's web user-interface I notice the security error saying "'server-A' did not encrypt this message". It's been happening presumably since when GMail added "security" option...AFAICT some emails back in 2016 I already see the error. Then, recently I switched the forward target from gmail to an address with googlegroups.com, which eventually delivers to the same gmail.com address. Then on GMail I/F, I stopped seeing the "security" error. In this case, is TLS covering end-to-end (i.e. in my case, sender -> server-A address -> googlegroups.com -> my gmail.com)? Or only to some extent?

Update 2022/05/22 In addition to the useful answers and comments, one of which I accepted as an answer, I found this article is exhaustive and useful (luxsci.com).

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    If the sender's MTA supports TLS and Server A supports TLS, then the connection from the sender's MTA to Server A is over TLS. At that point, that connection is torn down, the message is stored (in plaintext) on Server A. Then, Server A attempts to forward the message to Server B. If Server A supports TLS and Server B supports TLS, then the connection from Server A to Server B is over TLS.
    – mti2935
    May 4 at 17:49

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Email encryption is almost always* per-hop, between client to server or server to server. You can review the Received headers to see whether encryption was used on a given hop:

Received: from outgoing43.forevercar.com (outgoing43.forevercar.com [159.135.236.43])
    by bifrost.swynwyr.com (Postfix) with UTF8SMTPS id 7C2872199B
    for <gowenfawr@swynwyr.com>; Wed,  4 May 2022 12:07:20 +0000 (UTC)
Received: from <unknown> (<unknown> []) by
 api-n20.prod.us-west-2.postgun.com with HTTP id 62726c777a6ca2b6830651d2;
 Wed, 04 May 2022 12:07:19 GMT

Here we see no indication that the first (lowermost) hop used encryption, but given this was bulk mail and the sender was unknown it was probably "sent" locally for the first hop. The second (uppermost) Received header is marked with UTF8SMTPS which indicates SMTP secure (SMTPS) was encrypting that hop.

There are many ways a secure email hop may be inferred from the Received header:

with ESMTPS

with ESMTPSA

version=TLSv1/SSLv3 cipher=AES128-SHA bits=128

(using TLSv1 with cipher AES128-SHA (128/128 bits))

So if you look at your email headers you can tell where it got encrypted along the way.

*end-to-end email encryption is possible if the sender and recipient both choose to use it, however, the overwhelming majority of email sent is not sent end-to-end encrypted.

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  • On the other hand, only the latest Received header can be fully trusted, as the headers aren't signed and can be tampered along the path.
    – Esa Jokinen
    May 24 at 5:27

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