In the RecipientCollection of a Message, I need to add Recipient for sending an email to an arbitrary email id, which is neither a Business Partner, nor an internal user.
What values should I set for UserCode and UserType of the Recipient?
The DI API Reference document mentions UserCode as Mandatory:

whereas for UserType, the following are the valid values:

My addon will take the email id from the user as an input, and just wants to send mail to that mail id.