How exactly does email redirection work?

Question

Hi.

I'm trying to understand the technicalities of email redirection using rules. I set up a rule on Outlook.com to redirect emails coming from a particular domain. The first Received header in the resulting email is as follows:

Received: from BYAPR08MB4070.namprd08.prod.outlook.com ([::1]) by
 BYAPR08MB4070.namprd08.prod.outlook.com ([fe80::d156:3fc3:81fe:c917%5]) with
 Microsoft SMTP Server id 15.20.7386.017; Wed, 13 Mar 2024 17:17:11 +0000

What does the [::1] mean exactly?

The reason why I'm asking is because someone set up a rule, as far as I can tell, to redirect my email sent to a third party, so that the email never reached the intended recipient. For technical reasons I'd like to understand whether this occurred on the same physical server, and the same virtual server, if applicable, as would handle the final delivery of the email in the intended recipient's Inbox folder. Or, did the redirection occur via a different physical server or virtual server as what would handle the final delivery to the intended recipient?

Thank you for your assistance.

Bob