This warning message usually happened when your admin enable "My Domain" feature, but still allowed user login from https://login.salesforce.com
As admin, you can check this from Setup | Domain Management | My Domain, and look for Redirect Policy under My Domain Settings.
If Redirected to the same page within the domain is selected, users are immediately sent to the new URL, without notification.
If Redirected with a warning to the same page within the domain is selected, users briefly see a warning message before being redirected to the new URL. The warning gives users a chance to change their bookmarks and get used to using the new sub-domain URL. You can’t customize the message.
If Not redirected is selected, the user gets a “page not found” error. Eventually, you want your users to use only sub-domain URLs, but it’s a best practice to use Redirected with a warning to the same page within the domain for a short time after you deploy your sub-domain so that users can get used to the new URLs.
- My Domain
- Set the My Domain Login Policy
- What are the differences between the redirect policy options?