Now that almost all users use only a few email providers, is email content secure from interception? This related question was closed referencing a question about the security of SMTP IMAP, and POP:
How secure is email now?
However, things have changed over the years so that that question may no...
Your question seems to be focused on 'interception' of messages, while they are in transit. But, are you concerned about the providers that you mentioned (e.g. Google, Fastmail) 'turning evil', and accessing your email messages while these messages are at rest on the providers' servers?
It is very unlikely that independent providers use some custom protocol unlike SMTP to deliver mail between each others. Also, SMTP can already be used to achieve end-to-end security between providers, so no need to invent something else to get this security property.
basically the non-secure fallback is still there for most... Outlook:"For receiving mail into Exchange: If the sending server does not support TLS, or if the TLS negotiation fails, Exchange Online will still accept messages unencrypted and without TLS (provided the sending server’s configuration allows that). For sending mail from Exchange: For outbound email, if the receiving server does not support TLS (does not advertise the STARTTLS Verb), Exchange on-premises and Exchange Online will send email without TLS (provided TLS is not forced on the send connector or outbound connector)."
You can force TLS for certain recipients... see here: learn.microsoft.com/en-us/purview/… Unless you know otherwise, e-mail should be considered insecure by default because the fallback to non-secure protocol could be in place.
You would think that in 2023, major email providers (such as gmail, outlook, etc.) would enforce STARTTLS (with strict certificate checking) to other major email providers, to ensure that messages are encrypted in transit between these providers, and to prevent STARTTLS downgrade attacks. It would seem only natural, as most major providers have been using STARTTLS for years now. But, after a fair amount of searching, I am not finding anything to that effect.
@schroeder, in the thread you linked to you seem to say that secure e-mail never encrypts the e-mail content/message at all? Even if it doesn't fall-back to plain-text, non-secure protocol?? Did I read that right? I always thought the whole point of secure SMTP was to encrypt the e-mail.
sry, that was Steffen Ullrich... I'm questioning this line in his answer "even if every mail server involved uses TLS only the connection between the servers is encrypted but not the mail itself. This means that on every server involved in the delivery the mail is available as clear text."
I guess this means it's protected during each hop, but not necessarily from each node it's hopped to... It's going to allow any cert because it can't know via e-mail address which server is actually the destination. (I think I get it a little bit... bottom-line is even secure SMTP is not secure from MITM as is, and you'd have to use additional measures if you use SMTP and need the contents to be secure.)
Discussion on question by Croad Langs…
Discussion on question by Croad Langs…