When using telnet to create a test Postifx email, error 451 4.3.0 <firstname.lastname@example.org>: Tempoary lookup failure appears.
root@example ~ $ telnet example.com 25 Trying 192.168.0.24... Connected to example.com. Escape character is '^]'. 220 localhost ESMTP Postfix (Ubuntu) helo server 250 localhost mail from: email@example.com 250 2.1.0 Ok rcpt to: firstname.lastname@example.org 451 4.3.0 : Temporary lookup failure rcpt to: jeremy@example 451 4.3.0 : Temporary lookup failure rcpt to: jeremy@localhost 451 4.3.0 : Temporary lookup failure quit 221 2.0.0 Bye Connection closed by foreign host.
To identify the cause of this issue, on the Postfix server, use try sending a test email to root@localhost.
rcpt to: root@localhost
If the error appears when using root@localhost, there is likely some issue with the configuration of Postfix in the /etc/postfix/main.cf file. View the mail logs to see if there are events that help to identify the issue with the Postfix configuration.