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.