Can someone explain the setting 'Include original sender in email headers'

[u/RATTLECORPSE]

In settings, SimpleLogin has the option:

Include original sender in email headers

SimpleLogin forwards emails to your mailbox from the reverse-alias and not from the original sender address.
If this option is enabled, the original sender addresses is stored in the email header X-SimpleLogin-Envelope-From and the original From header is stored in X-SimpleLogin-Original-From. You can choose to display this header in your email client.
As email headers aren't encrypted, your mailbox service can know the sender address via this header.

I need help understanding this. Especially the part where it says 'As email headers aren't encrypted, your mailbox service can know the sender address via this header.' I am using SimpleLogin with Proton. So does this mean that checking 'Include sender address in email headers' make it less secure?

[u/nethack47]

Some setups don’t need complete privacy.

I have my own domains, if I ran a business address I might have another domain for communication but with references to the original sender for verification etc.

Email is a mishmash of many messy standards and solutions. I recommend looking into what qualifies as an address globally and have that in the back of your head :)

[u/RATTLECORPSE → u/nethack47]

But what exactly does it mean 'As email headers aren't encrypted, your mailbox service can know the sender address via this header.'. Since Proton claims to be zero knowledge, does this mean Proton can or cannot see the sender?

[u/gvasco → u/RATTLECORPSE]

Because of the way e-mail works and it verifies the source domain against DNS registers, the sending address is swapped by a reverse proxy one. This ensures that e-mails pass attestation when checked by the main provider and end up in your inbox but also that you can reply via the alias by replying to the e-mail address included without fearing leaking your main address.

Think of it like a VPN (more of a proxy really tbh) but for e-mail, e-mails are received by your proxy provider and forwarded to your e-mail provider. If you want outgoing e-mails to ve forwarded the same way then you can't send them directly from your e-mail account to the original sending address, instead the sending address is replaced by something that the proxy provider knows what original address it must send the e-mail too and what alias it must use to put on the e-mail headers.

By default the sending address is this new reverse alias and the actual sending address isn't included in the headers. This option forces it to be included so that the e-mail may be replied to directly if so wished instead of the reverse proxy.

[u/timewarpUK → u/gvasco]

Where does it mention replies? It just says two new headers will be added starting X-SimpleLogin- unless I'm misunderstanding?

[u/gvasco → u/RATTLECORPSE]

Proton is zero-knowledge. At rest all your e-mails are encrypted and only accessible if you can get into the account, and for the body of any e-mails encrypted with your public key.

Any e-mails that come into proton unencrypted can be seen/scanned by proton before they are encrypted and stored.

Despite that, because of the workings of e-mail and being an old protocol all e-mail headers are always visible to any provider. The reverse alias replacement means that the provider can't directly see who the original sender was. (Although if you look at them they are easy to reconstruct tbh). If you choose to include the original sender in the forwarded header than proton will be able to see it in the headers.

This is just an inherent "weakness" in e-mail due to how old it is and difficult to make any non-breaking changes that would maintain compatibility throughout adoption and implementation of anything else a bit more private.

However, any such privacy changes would also likely make spam filtering much more difficult too.

[u/RATTLECORPSE → u/gvasco]

Thank you for the in-depth response there!

[u/timewarpUK → u/gvasco]

Yeh seems a bit of a nothing burger since the email is already like user_at_example_com_abc123@simplelogin.co

[u/gvasco → u/timewarpUK]

No-one besides you will see it though, the only ones who can is your e-mail provider.

[u/timewarpUK → u/gvasco]

Isn't that the same case as with the header?

[u/gvasco → u/timewarpUK]

Well yeah, this is all info that is on the header, but the header is modified by the e-mail relay server.