how can I know the email and the password I use to send and receive? Thanks

For full-service email, you may set up email accounts and password in 'Accounts' section in the control panel. You may refer to https://www.dynu.com/Resources/Tutorials/EmailServices/EmailServiceSetup/FullServiceEmail to set up the service. To set up email client to send and receive emails, you may refer to https://www.dynu.com/Resources/Tutorials/EmailServices/ClientConfiguration

For email forward service for yourdomain.com, you can set up an email address that you already own such as yourname@gmail.com to be 'Catch-all Email Address', all emails sent to *@yourdomain.com will be forwarded to yourname@gmail.com. You may refer to https://www.dynu.com/Resources/Tutorials/EmailServices/EmailServiceSetup/EmailForward for more details.