Hello @JenniferH15 , Ultimately, we send using our own servers. So any IPs or Message IDs/Envelopes will all connect back to CTCT. However, when dealing with emails (especially when using a 3rd party like us), receiving severs will still see who the From Address is, since that's the Org/Business that is sending the email.
Different levels of authentication might affect what a contact sees in their header, but ultimately the email will still appear as coming from you, and the receiving servers will still know that your email is the one being used to send the email. For example if you're sending with self-authentication, the servers will read your DKIM key and absolutely confirm it's an email coming from someone associated with your organization, rather than someone potentially disconnected from the organization trying to send it. Some receiving servers can be quite strict on who is sending the email, which is why self-authentication with your own domain tends to result in the absolute best deliverability. However this is a fairly moot point if you're a smaller org or business just sending to peoples' personal emails, rather than corporate or government addresses with strict IT security systems.
For a more in-depth discussion on this, I'd definitely recommend reaching out to our Deliverability / Review team directly. However this is the gist of how it works as far as the receiving server side.
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