Error The email must be a valid email address

Hi, I am installing self hosted invoice ninja using Ubuntu 20.04 with invoiceninja v 5.4

But I am kept getting error ‘The email must be a valid email address.’ whereas my email is a valid email address. I registered in invoiceninja portal using this email but unable to complete the task. Screenshot below.

Appreciate if expert can post what is going wrong here.

Thank you

The system uses this package

to ensure that a valid email address has a mail server configured. It may be that you are using an alias which the system cannot lookup?

I am not using alias, it is the actual email address.