Some mail protocols support streaming operations. This means that the MTA can issue more than one operation at a time and wait for replies to each operation to arrive in batches. The streaming keyword controls the degree of protocol streaming used in the protocol associated with a channel. This keyword requires an integer parameter; how the parameter is interpreted is specific to the protocol in use.
Under normal circumstances, the extent of streaming support available is negotiated using the SMTP pipelining extension. As such this keyword should never be used under normal circumstances.
The streaming values available range from 0 to 3. A value of 0 specifies no streaming, a value of 1 causes groups of RCPT TO commands to stream, a value of 2 causes MAIL FROM/RCPT TO stream, and a value of 3 causes HELO/MAIL FROM/RCPT TO or RSET/MAIL FROM/RCPT TO streaming to be used. The default value is 0.