From which address the emails are sent?

The emails are sent from the platform via the email domain: expoplatform.com (Default)

But if you want to use your own email domain, that is also possible.

Go to Event Setup → General → Settings

Scroll down → Enter the email domain you would like to use for the emails sent out from the platform

Note: The email domain needs to be verified by AWS (Amazon Web Services) and to complete the verification some changes needs to made in your domain management console.

Expoplatform team will share the file with instructions on the TXT records and CNAME records that need to be set up on the domain. Once the validation is done, you will be notified about the same by the team.