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:
However, things have changed over the years so that that question may not be relevant in all cases:
When for example I send email using gmail to another gmail user, who reads their email using google's website or app, I can imagine that perhaps none of those protocols are used (here I don't want to debate if such email counts as "real email": for the purposes of this question, this is still email) and all steps in transmission are secured against interception of email content (defined as, say: sender, recipient, subject line, message body, and attachments)
When for example I send email from fastmail's website to a gmail user, perhaps the same is true: i.e. perhaps in practice fastmail has secure transmission arrangements in place with Google
With that in mind: Is email in practice (i.e. using common methods of access such as gmail and fastmail, without necessarily directly using SMTP/POP/IMAP) now secure from interception in 2023?
I'm referring here to interception other than by the email providers themselves: I'm talking about interception by 3rd parties (5th parties?? i.e. not the sender, not the recipient, nor the email providers of the sender or recipient).
