Are you receiving a Bounce back (NDR - Non-Delivery Report) when sending an email message? We have provided a list below with basic knowledge on what the codes actually mean. You will need to look at the NDR received, view the code to compare it to the list below.

NDR List

4.4.2 - The server started to deliver the message but then the connection was dropped. The sending server is configured to retry automatically.

4.4.7 - Problem with a protocol timeout, for example a message header limit. Check receiving server connectors.

5.1.0 - Sender denied. Often seen with contacts. Verify the recipient address.

5.1.2 - SMTP; 550 Host unknown. An error is triggered when the host name can’t be found. 

5.1.3 - Invalid recipient address. Another problem often seen with contacts. Address field maybe empty. Check the address information. 

5.1.4 - Destination mailbox address ambiguous. Two objects have the same address, which confuses the Exchange 2007 Categorizer.

5.2.1 - Mailbox cannot be accessed. The mailbox may be offline, disabled, or the message has been quarantined by a rule.

5.2.2 - The recipient has exceeded their mailbox storage quota.

5.2.3 - Message too large. Recipient cannot receive messages this big. Try re-sending the message without the attachment.

5.3.4 - The message too large. Check the limits on both the sender and receiver side. There is likely a policy in operation.

5.4.4 - Invalid Arguments. No route exists for message delivery. Have recipient check their MX Records.

5.5.3 - Too many recipients. Try resending the same message to fewer recipients.

5.5.4 - Invalid domain name.

5.7.1 - Delivery not authorized. Sender is not authorized to send to the recipient. 

5.7.1 - Unable to relay. The sender is not allowed to send messages to the recipient.

If you would like further assistance you can log a ticket to our support staff. Make sure to include the attached headers and original message that was received.