SMTP has headers that are visible to the email provider while the email is in transit. The absence of logs, etc., goes far to mitigate this threat. As an added benefit, there is a lot of anti-spam functionality that can be used with only this metadata available, and only ephemerally. I don't think it's true that zero-knowledge at rest means inability to provide common email provider value adds like anti-spam.

To add to the above post, the headers have to be unencrypted, else they'd have no method to actually send the email. If it were encrypted, they'd have no idea what the address was, or who to bounce it to if the address doesn't exist.

Email headers contain a lot of information. It has the various email addresses, servers involved, ip addresses, time stamps, subject, priority, and things like that.

The body of the email is the only part that gets encrypted when encryption is in use.

You are conflating the SMTP envelope and the mail headers. The SMTP envelope is necessary for delivery, the mail headers are not (the addressee information is duplicated)

Thanks for the correction. I had originally typed 'meta data' but thought headers were the same thing. Thanks!

Whatever happened to filtering client side?

Not much. It's as bad now as it was a decade ago.

In other words, about as good as gmail but with better control over false positives.

For me, Gmail solves the spam problem. I don't even remember the last time I've had a false positive or a piece of spam that made it to my inbox.

My entire family uses Gmail (our domain is hosted via Google Apps) and as far as I know, it's been very close to perfect for all of us.

Well, do you really check your spam folder all the time? There might be false positives that you miss.

There are also false positives than never make it into your spam folder - that just get dropped silently. (I have a friend who's company's GSuite email _regularly_ does this...)