How can I use the Mail System module to send one e-mail differently than others? Problem

I would like to configure Mail System to use a different mailing class (one that does not format e-mail with Mime Mail) for the "welcome (no approval required)" e-mail.  Mail System supports adding a New Setting, which takes a module and a key.  For module I selected Core: user module, but what should I select for the key?

Background

On my D7 site, I am using the Mail System module to manage e-mail.  By default, everything is formatted by the Mime Mail module and sent with the Smtp module. 

This is great for every message except the "welcome (no approval required)" email.  The reason is that I am using Logintoboggan and that e-mail contains the link to validate the account; when it is formatted with HTML (processed by the Mime Mail module), then sometimes users on mobile phones are not able to click the validation link.
These are the keys uses by the core user module:


'register_admin_created': Welcome message for user created by the admin.
'register_no_approval_required': Welcome message when user self-registers. 
'register_pending_approval': Welcome message, user pending admin
approval.
'password_reset': Password recovery request.  
'status_activated': Account activated. 'status_blocked': Account blocked. 
'cancel_confirm': Account cancellation request.
'status_canceled': Account canceled.