Sitefinity defaults the sender for registration emails to “firstname.lastname@example.org
”. To change it go to Administration > Settings > Advanced > Security > Membership Providers > Default > Parameters > Create new.
Once the page loads set the Key to successfulRegistrationEmailAddress and the value would be the email address sender you want to send from. (email@example.com).
You can also change the “name” of the sender in the "Portal Registration" widget itself, under the Advanced properties of the Portal Registration widget or Sitefinity Registration widget, set the EmailSenderName property to the name that you want set in the email.
More information on this can be found at the Sitefinity KB article here: https://knowledgebase.progress.com/articles/Article/Configure-Registration-Widgets-Email-Sender-Name