If you receive this error when adding a domain:
Unable to send notification: mail() failed: SMTP server response: 550 Requested action not taken: mailbox unavailable or not local
then you need to add a valid mailbox to your Plesk admin setup. To resolve this issue log into Plesk and click on:
Edit (under personal information)
Replace the email address with a valid, working address.
Keep in mind this error shouldn’t prohibit you from creating a site, it is just notifying you that the “site created” email will not be sent.