Private message draft cannot be saved with only one filled field

As a user I want to be able to save my mail to Draft without validation (just 1 field should not be empty, Save button is disabled if all fields are empty)

Steps to reproduce:

Scenario 1
1. Go to Inbox page
2. Click 'Compose' button
3. Input any text into 'Message content' textbox
4. Click 'Save' button

AR: The message isn't saved, errors about Subject and To fields appears
ER: The message is saved without validation if 1 message is not empty

you also can not save a draft with just To or just Subject filled out field