Specify the user name to use for the envelope From: and header line From: addresses. The item_address and item_length fields specify the address and length of a text string containing the user name.
Use this item code when the envelope From: address is a local address.
If the envelope From: address is not a local address, then the MTA_ENV_FROM action should be used.
If this action and the MTA_ENV_FROM actions are not specified, then the user name associated with the current process will be used.
On UNIX, the process must have the same (real) UID as the root or mta account. If the process lacks sufficient privileges, the MTA_ACCESS error will be returned.
Do not use this item code in conjunction with the MTA_ENV_FROM item code.