>I went to the link which is on mailchimp-sso.com and entered my credentials which - crucially - did not auto-complete from 1Password. I then entered the OTP
These clever phishing schemes fool even the tech-savvy computer users like Linus Tech Tips, Matthias Wandel, and Troy Hunt. This problem should be "low hanging fruit" for A.I. to identify and help users. E.g. Apple Intelligence on iPhones to scan the emails and text messages for red flags and some something similar in desktop email clients like Mozilla Thunderbird.
Anyways, comparing the WHOIS info for legitimate vs fake:
Domain: mailchimp.com
Registered On: 2001-06-29
Registrar: MarkMonitor Inc.
Country: US
Domain: mailchimp-sso.com
Registered On: 2025-03-24
Registrar: NICENIC INTERNATIONAL GROUP CO., LIMITED
Country: AE (United Arab Emirates)