The problem

You want to know whether an email transmission was shared with Microsoft or Google so you can act accordingly.

tx (n/a)

Doing an MX lookup before sending a msg will often give answers (e.g. if the recipient simply uses MS or Google without using a 3rd-party email firewall like barracuda or a forwarder). So an MX lookup can only be either CAGEMAFIA-positive or it can be inconclusive.

In any case, the proposed filtering tool does nothing to help with sending.

rx

For receiving, you have much more certainty of couriers involved but inspecting headers manually is tedious and error prone. E.g. if “outlook” appears in the msg id header it likely just means Outlook was the MUA not the network used.

The fix and concept

Just as spamassassin filters a msg to work out whether it’s spam, we could use a filter tool to work out which networks an inbound email traversed. It could add a header like: X-Path: Microsoft → Spamgourmet → Disroot You configure your mail client to highlight baddies in this header, like “Microsoft”.

So suppose you originally did an MX lookup which revealed “barracuda”, thus inconclusive on whether the other person uses a surveillance capitalist. You decide to take a risk and reveal your email address to the other party. Then they use it to email you which reveals they use MS in the routing. You can then demand that they erase your email address (under GDPR or anti-spam protection in some regions).

You could easily write another filter to add the sender and X-Path to a local database. You could also publicise the information to help others. E.g. microblog that “Gov agency X uses Microsoft for their email (masked by baracuda) -- snail mail recommended”.