When sending an email, the following failure is received where "firstname.lastname@example.org" is the email address you are sending an email to, and "email@example.com" is the email address that sent the email:
SMTP error from remote mail server after RCPT TO:<firstname.lastname@example.org>:
450 4.1.8 <email@example.com>: Sender address rejected:
Domain not found
This failure occurs when the remote mail server cannot resolve the sender's domain to an IP address.
Ensure the sender's domain resolves to an IP address.