From V13 email domain is not mandatory. You can directly add email account.
Try configuring your email account in other email clients(eg gmail app). This is to make sure your email account is allowed to make smtp. Once successful in this step then come back to frappe and configure the default outgoing email.
Once problem in frappe V14 is, the default outgoing is not considering the below setting
Try changing the administrator’s email address to the email address you configured for default outgoing.(from admin@example to your outgoing email address.)