When using telnet to create a test Postifx email, error 451 4.3.0 <username@domain.com>: 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: test@bogus.com
250 2.1.0 Ok

rcpt to: jeremy@example.com
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.