Setting the Sender: Address

A Sender: address specifies who sent the message and is set with the setSender() method of the message.

To set the Sender: address:

Call the setSender() method on the Message.

The Sender: address is visible in the message headers and will be seen by the recipients.

This address will be used as the Return-Path: unless otherwise specified.

Note: If you set multiple From: addresses then you absolutely must set a Sender: address to indicate who physically sent the message.
Note: You must not set more than one sender address on a message because it's not possible for more than one person to send a single message.
$message->setSender('your@address.tld');

NOTE: Documentation for version 3 is in the wiki.