Adding multiple external email domains in ms365 business Premium?

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Refer below steps:

1. Add Custom Domains:

   • Sign in to the Microsoft 365 admin center
   • Go to Setup
   • Under Get your custom domain set up, select View > Manage > Add Domain
   • Enter the new Domain name that you want to add
   • Follow the prompts to sign in to your domain registrar and verify ownership of the Domain
   • Choose the services (such as Exchange Online, SharePoint, etc.) that you want to associate with the new Domain
   • Complete the setup process

2. Add Email Aliases for Users:

   • In the admin center, go to Users > Active users
   • Select the user for whom you want to add an alias
   • Click on Manage username and email
   • Add the new email alias in the Username field.
   • Select the appropriate domain from the drop-down list
   • Choose Add