X-Google-Smtp-Source

After some years (this blog was known as blog.postmaster.gr in the past afterall) I had to do some digging into an email’s headers. I couldn’t find what I was looking for, but there was a header inserted by the Google Workspace system named X-Google-Smtp-Source. It had a base64 encoded value. Decoding it did not produce any immediately meaningful result, so I dug a bit more. Searching for this header brings pages that repeat the following information from Google:

X-Google-Smtp-Source: This header shows the IP address of the computer that sent the email.

https://support.google.com/mail/thread/230509179/need-to-know-email-sent-recieved-times?hl=en

This does not provide much information. I tried to look for more, and even asked ChatGPT and Bard but they too came up with nothing helpful. Until I reached the following mail thread:

:
These headers are typically base64 encoded encrypted serialized protocol
buffers. Without the key, you won’t get anything out of them, and the
keys rotate on a schedule. For what’s actually in them, it’s probably
overkill, but better safe than sorry.

They also don’t contain the IP address.
:
As Grant pointed out, we consider the IP address of the user to be PII and
do not share it in most cases.

https://www.mail-archive.com/mailop@mailop.org/msg08419.html

Notice also the contradiction: the header seems not to carry the originating IP address. It seems that at a point in time this was indeed carrying the source IP, but something (maybe legislation?) changed and the contents of the header changed too, without the documentation being helpfully updated.