Mail in MyTrader is sent to devices, not to people or to trading accounts. Each device is identified for mail by an 8-character code such as DMZLXCVG. This ID is shown in the send-mail window in MyTrader.
A widget, script, or UDIX can easily send a message to its own device - the current user - without needing to know the device's mail ID. But the following two things are entirely deliberate:
A widget, script, or UDIX cannot programmatically discover the mail ID of its own device
There is no way of discovering the ID of another device without that device's user looking it up, in the mail-sending dialog, and choosing to tell it to you
It is also possible to send mail into MyTrader from outside the platform, from any environment which can make HTTP requests.