When you configure a messaging port, you can select a primary transport and/or a backup transport. Transport services are defined in the BizTalk Server documentation as:
"...a set of services that includes network protocols and application
integration components (AICs). BizTalk Server 2000 supports a core set
of transport services. This enables the server to send documents to
organizations or applications whether or not the applications are capable
of communicating directly with the server by using a COM interface.
BizTalk Server 2000 supports the File, HTTP, HTTPS, and SMTP network
protocols and Message Queuing. Transport services are also referred
to as transport components."
When BizTalk Server processes a document through a messaging port, it first uses the primary transport to deliver the document. If for any reason this delivery mechanism fails, the document is sent to the Retry queue. Then, BizTalk Server tries to deliver the document again according to the configuration of the channel. After BizTalk Server exhausts all of these attempts, it uses the configured backup transport to deliver the document. The cycle of configured attempts recurs and when they have all been exhausted, the document is sent to the Suspended queue.