451 4.4.0 SMTPSEND.SendingError; Error while sending message

Question

Hi All,

We have an application server which is pointed to one of the Exchange server in the environment to relay the mail to the recipients from external domains.

The mails from the application server were successfully sent via exchange servers to external domains, but application team has raised a complaint where some of the messages were not delivered to the external recipients.

We have asked the application team to share the message id and session id of that failed message transaction, then we had checked in protocol logs of the receive connector, but we don't found the details given by the application team in the protocol logs of the receive connector. So we suspect the message has been dropped at the network layer present in between Application server and Exchange server

Though the failed message transaction details provided by appplication team is not found in the protocol logs, We were able to see the below errors in the protocol logs for the SMTP connection coming from the application server

INFRAEXCH02 - Exchange server Name
10. 50.108.11 - Exchange server IP address
10. 39.110.103 - Application server IP address
Port number - 25
Error messages given below
Message or connection acked with status Fail and response 451 4.4.0 SMTPSEND.SendingError; Error while sending message
451 4.7.0 Temporary server error. Please try again later. PRX5
Remote(SocketError)

ECV2022122811-2.LOG:21240:2022-12-28T11:42:12.735Z,INFRAEXCH02\Anonymous-Relay,08DAE5FB96FE2B2E,12,10.50.108.11:25,10.39.110.103:23056,*,,Message or connection acked with status Fail and response 451 4.4.0 SMTPSEND.SendingError; Error while sending message
RECV2022122811-2.LOG:21241:2022-12-28T11:42:12.735Z,INFRAEXCH02\Anonymous-Relay,08DAE5FB96FE2B2E,13,10.50.108.11:25,10.39.110.103:23056,>,451 4.7.0 Temporary server error. Please try again later. PRX5 ,
RECV2022122811-2.LOG:21242:2022-12-28T11:42:12.735Z,INFRAEXCH02\Anonymous-Relay,08DAE5FB96FE2B2E,14,10.50.108.11:25,10.39.110.103:23056,-,,Remote(SocketError)

Any idea what could be causing these error messages found in the protocol logs for the smtp connections coming from application server ?

Answer 1

Hi @Nithyanandham Singaravadivelu ,
"451 4.4.0 SMTPSEND.SendingError" is the content of the SMTP error report, there is no information about this error in the Microsoft documentation. The same email could be sent again successfully, which means the problem does not occur on the Exchange server side.

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Answer 2

@Jame Xu-MSFT

Thank you for your response

Question : According to the error report, the problem is related to the network connection between servers. Could a failed email be resent after a period of time and be sent successfully?

Answer : Yes the application team can able to resend that same mail in the next attempt

Any Microsoft Article which says the below error code is related to Network Problem ? It would be really helpful if you could share that with me.

Message or connection acked with status Fail and response 451 4.4.0 SMTPSEND.SendingError; Error while sending message

Answer 3

Hi @Nithyanandham Singaravadivelu ,
According to the error report, the problem is related to the network connection between servers. Could a failed email be resent after a period of time and be sent successfully? You could check whether the network between servers is smooth by ping test or other methods.

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