The TRANSMIT startup exit (INMXZ01R or INMXZ01) receives control after the TRANSMIT command processor has parsed the command and built the addressee list, but before it has done any transmission-related allocations. Its primary uses are to:

• Control to which nodes and user IDs transmissions are sent. The exit has access to and can change the information in the addressee list, which specifies the target node and user ID of each addressee. If the TRANSMIT command specified a nickname, the addressee list also includes the nickname and :NAME and :PARM tag information from the NAMES.TEXT data set. The exit can also identify the sender, and thus restrict network use differently for different users.
• Prevent or force an acknowledgment to be sent to the issuer of TRANSMIT when the transmission is received. The exit can prevent or force acknowledgment individually for each entry in the addressee list.
• Perform initialization tasks You can use the exit to obtain storage for the exit-to-exit communication word, allocate data sets, open files, or do other set-up processing for the other TRANSMIT exits.
• Enable users to transmit data types other than those supported by TRANSMIT. The exit can detect when an unsupported data type is being processed. It can then allocate a temporary data set, and invoke an installation-specified utility to convert the data to a format that TRANSMIT recognizes, and copy it into the data set. The exit passes the name of the data set to TRANSMIT for processing. The exit can also pass up to ten local control records, which are sent with the transmission. You must also write either the RECEIVE data set pre-processing exit (INMRZ11R or INMRZ11) or the RECEIVE post-prompt exit (INMRZ15R) and the RECEIVE data set post-processing exit (INMRZ12R or INMRZ12) to process the unsupported data. See Functional description for more information about the data types TRANSMIT and RECEIVE do not support, and how to use exits to process that data.