The error given in the bounceback is "No such user here" and it is referring to the email address firstname.lastname@example.org.
That email account must exist on your account in order to send a mail with that address as the sender. You can simply create that email account via the cpanel and then you should be able to send the email without further error.
I hope this answers your question. If you have any more questions or information specific to the issue please leave a comment below so we can further assist you.
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