Universal Containers has multiple Salesforce instances where users receive emails from different instances.

My Domain is a feature that allows you to personalize your Salesforce org with a subdomain within the Salesforcedomain. For example, instead of using a generic URL like https://na30.salesforce.com, you can use a custom URL like https://somethingReallycool.my.salesforce.com10. My Domain should be enabled in Salesforce as a prerequisite for the following reasons:

My Domain lets you work in multiple Salesforce orgs in the samebrowser. Without My Domain, you can only log in to one org at a time in the same browser.

My Domain lets you set up single sign-on (SSO) with third-party identity providers (IdPs). SSO is an authentication method that allows users to access multiple applications with one login and one set of credentials. With My Domain and SSO, users can log in to Salesforce using their corporate credentials or social accounts.

My Domain lets you customize your login page with your brand. You can add your logo, background image, right-frame content, and authentication service buttons to your login page.