The document discusses mail flow in Exchange Online. It covers mail flow rules (also known as transport rules), message tracing, accepted domains, remote domains, and connectors. Mail flow rules allow administrators to identify and take action on messages as they flow through the organization. Message tracing allows administrators to view details about a specific message's journey. Accepted domains are domains that users can send and receive mail from. Remote domains control the types of messages sent outside the organization. Connectors are only needed in some circumstances to control mail flow between Office 365 and other email servers or partner organizations.