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Proofpoint, Inc.

Common Error Codes found in Message Details

Situation Email delivery status is displaying an error code due to bounced or deferred messages and Inbound error messages.
Solution These Error Codes can provide clues that can assist an admin in troubleshooting and correcting issues with their mail system.

Sending logs may show the error "Failed to Connect" when handing off messages to Proofpoint servers.

Proofpoint uses a pool of servers to accept messages. If a sending server happens to hit a server that is already busy it will give the error and then try the next sever in the pool. This is standard behaviour for a large scale messaging system and the reason we have a pool of servers to accept mail.

This error may cause concern to those viewing sending logs but is a normal part of everyday connections to a large pools of servers.

Proofpoint continually monitors our pool of servers and increases capacity when we see these errors exceed specific normal expected threshholds.

Should there be any issues accepting a message, a NDR or deferral will indicate an actual issue with handing off a message.

Log Summary from the Connections Details View

From the logs, you can click on the Log Details Button and view the Per Recipient & Delivery Status section. The delivery status often shows error codes explaining why a message shows as bounced or deferred.

Any time you see the error This message was rejected by its destination for reasons outside the control of Proofpoint Essentials, and got returned to the sender, it means that this is outside our control, and this issue must be with the Customer/Recipient server accepting the message.

421 4.3.2 Refused to talk to me

Proofpoint cannot make a connection to the mail server. This could be due to multiple issues, but ultimately the server is closed off from making a connection. 

452 4.3.1 Insufficient system resources

This normally means that the recipient/customers server doesn’t have enough resources to accept messages. Customer/recipient needs to resolve this issue before we can deliver the message.

550 5.7.1 : Recipient address rejected

There are two possible issues here. See below for marked as invalid. Or, the email address does not exist in the Proofpoint Essentials system. Also, it would give a possible error of user unknown.

550 5.7.1 User email address is marked as invalid

This issue has to do with the Proofpoint Essentials SMTP Discovery service. If a user was not formally created into the system and is in the invalid list, this error will occur.

550 5.1.0  sender rejected

The senders IP address is rejected due to a Blocklist/wrong SPF. This is outside the control of Proofpoint Essentials

550 5.7.1 Relay Access denied

We are a closed relay system. This means if a user is in the Proofpoint Essentials system we will deny access to any e-mail that is not registered. More information on this error can be found here.

connect to[xx.xx.xx.xx]:25: No route to host

This error is caused when Proofpoint attempts to do an MX lookup on the domain and no information is found. This could be a DNS issue with the domain owner / DNS provider or an issue with the Proofpoint DNS servers no having updated / correct MX information.

Check some common DNS lookup sites ie. or to see if a valid MX is currently registered for the domain. If it is, then you will need to contact Essentials Support to have us check our Proofpoint DNS servers for valid MX information.

For more information and understanding on error codes please visit