Architecture that facilitates transport high availability for messaging services by providing the ability of a receiving entity (e.g., receiving message transfer agent (MTA)) to detect if a sending entity (e.g., sending MTA or client) is a legacy sending entity. When the receiving entity detects that the sending entity is a legacy system, by advertising transport high availability capability to the sending entity, if the sending entity does not opt-in to this capability, the receiving entity keeps the sending entity client “on hold”, that is, waiting for an acknowledgement (ACK) until the receiving entity delivers the message to the next hops (immediate destinations). This approach maintains at least two copies of the message until the message is successfully delivered (to the next hop(s)). Hence, if the legacy sending entity or the receiving entity fails, the message is still delivered successfully.