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.
Received: from smtpclient.apple (ptr. [ip])
by smtp.gmail.com with ESMTPSA id ...
for <...>
(version=TLS1_2 cipher=ECDHE-ECDSA-AES128-GCM-SHA256 bits=128/128);
Thu, 02 Jul 2026 ..:..:.. -0700 (PDT)
For me that IP belongs to apple, when using the iOS mail client. I presume Apple forces third party apps to use their API and then stuff like this happens. Guess that's also on google, not mitigating that issue, tho.
Have you tried a VPN? I wonder, if Apple manages to expose IP despite a VPN. They had issues with stuff like that before.
