Hi, Erymar Torres
Thanks for posting in the community.
You can first ask your customers to add a new login method:
To add a new way to verify your sign-in:
Microsoft account security info & verification codes - Microsoft Support
- Sign in to the Advanced security options page for your Microsoft account.
- Select Add a new way to sign in or verify, and then choose how you want to receive your security code.
Change your security info
Note: You can add up to 10 different ways to verify your sign-in, and you can choose from:
- Email address or phone number. It doesn't have to be yours - you just need to have access to it if you want to use it to verify your account.
- Authenticator apps, like Microsoft Authenticator, provide easier verification and allow you to sign in even if your verification device is offline.
- Secrets. Passwords are secure, phishing-resistant replacements for passwords, which means they can't be guessed or stolen. Sign in with your face, fingerprint, or device PIN.
- A security code will be sent to the new number or email for confirmation. Enter the code when prompted, and then select Next.
- Once you're done, you'll see the new option listed the next time you need a verification code to sign in.
After that, remove the previous sign-in verification method.
Removing a sign-in verification method - Microsoft Support
- Sign in to the Advanced Security Options page for your Microsoft account.
- Expand the verification options and select Remove.
- Select Remove again to confirm.
This should remove the alternate email.
Please feel free to let me know how it goes.
Best Regards, Zain.M - MSFT | Microsoft Community Support Specialist