Internal Email is shown in sent items but the receiver can not receive the email

Hello.

I deployed a test environment in lab with 3 vm computers. One is for AD server whose ip equals "10.204.166.43".  the second is an exchange 2016 server, and its ip is "10.204.166.34". The third is a test client computer which ip is"10.204.166.44". The dns server is all "10.204.166.43".

User test003(test003@zack.test) can send email to test004(test004@zack.test), and the email is shown in "Sent Items". But user test004 cannot see the email.

With Get-MessageTrackingLog shell command, I got log information like "EventId:FAIL, Source:SMTP, MessageSubject: Undeliverable".

May someone help me?

Thanks a log!