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I also just tried this as well, sending an email from a Migadu-based account to one at both Gmail and MXRoute using Mail.app under macOS 15.7.7. Neither included any private IP address info I could find in either headers or raw source. That would be a good leak to know about and as sibling comment said saagarjha definitely knows their stuff, so any tips to replicate would be appreciated.
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Is this from iOS? I just tested from MacOS, and the only IPs were for the transit and auth servers.

I'm not actually doubting it. saagarjha knows his stuff. I just don't see it, so maybe I'm holding it wrong.

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Same, I remember it being in the headers long ago but don't see it now. macOS Tahoe, Mail.app client, iCloud email sending to itself or to my Gmail, both set up in default ways, IPv6 disabled
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