Retaining internal Exchange email headers
Applies to: Exchange Online, Exchange Online Protection
As you create connectors, you have the option to either retain internal Exchange email headers or discard them when a message passes through a connector. We recommend keeping them because this can help Exchange servers to communicate valuable information.
Decide whether to retain internal email headers
When you choose to retain the Exchange email headers, you are indicating that the information contained in each message header should be preserved and it should continue to be associated with the message while it is being transferred by a connector. You're not required to preserve the header information with email.
Email headers are most helpful if you have a hybrid deployment, which is mailboxes in both Exchange Online and your email server (also called an on-premises server). In most cases, when you use the Hybrid Configuration Wizard to set up your hybrid deployment, you won’t need to make any changes to connectors in the Exchange Admin Center. For more information about configuring a hybrid deployment, see Exchange Server 2013 Hybrid Deployments. Even if you're not using a hybrid deployment, the header information might be useful to other email systems eventually, and for this reason, we recommend retaining the headers.
What is an Exchange email header?
The header is a record or log of data about the message but it is not the body of the message itself. When Exchange processes messages, it's a complex operation that is broken into a set of steps. For example, Exchange can keep track of each server that a message passed through and when. Exchange can verify whether messages meet your business requirements if you have any kind of filtering set up, whether messages contain an accurate destination address, even if a message has been checked for malware, and more. There are quite a few steps involved and Exchange breaks these down into discrete tasks. As each task is started or completed, a small amount of information can be added to the message header. The information describes the condition of the message as related to all the different tasks. The message header travels along with the original message, but it's not usually shown to the recipient.