Unfortunately, they are not the only one with that bad practice. Google does too, for a long time already. (different service mails) This is an example from last security warning X-Received: by 2002:a05:6602:4742:b0:83a:a82b:f856 with SMTP id ... Thomas Am 21.01.25 um 14:38 schrieb Jordi Palet Martinez via ipv6-wg:
Hi all,
I’m wondering if I’m understanding this correctly and if so, if it is a good practice.
In email mesases received from IONOS, I see in the header:
X-Received: by 2002:ac8:5ac5:0:b0:46d:fd08:7c9f with SMTP id d75a77b69052e-46e12a462a7mr251941861cf.22.1737462364476; Tue, 21 Jan 2025 04:26:04 -0800 (PST)
Another example:
X-Received: by 2002:a05:622a:38b:b0:46c:7646:4a1e with SMTP id d75a77b69052e-46e12a62966mr243715391cf.13.1737416130721; Mon, 20 Jan 2025 15:35:30 -0800 (PST)
It looks to me like they are using 2002 as the IPv6 address of the email for the client hosted mail/domains?