Value in the 'To' field in the Case Feed Email Publisher

When sending an email from a Case Feed using the 'Reply' in the email publisher, the 'To' field for the email will populate based on the following criteria,

• When the last Email on the Case is Outbound, 'To' will default to all TO addresses from that last email, excluding any addresses the current User can send Case Email 'From' (e.g. Excluding Addresses listed in the From picklist when composing email from the Case). Using 'Reply All' will have the same result plus including any recipients in the 'CC' into the new message's 'CC' field.
• When the last Email on the Case is Inbound, 'To' will default to the FROM of that last email, again excluding addresses the current User can send Case Email 'From'.
• When using a Predefined setup in a custom Send Email action for the "To Recipient" the value is only expected to populate on initial email sent.
• The predefined set up on custom email action won't apply on cases created from Email to case , as the first email is already received on the case which will update "TO" address from the email received.
• The behavior of the 'To' field is also the same when creating new emails in Lightning Case Feed Publisher
  
  

Note:

• If no address is found based on the above, or if all addresses that are found are excluded based on being addresses that Users can send case emails from, then the Email Publisher 'To' field will not default to any value.  When using an Email Quick action, if there are no related email messages, and no predefined values, the 'To' address will default to blank.
• The QuickActionDefaultsHandler interface  allows control over the defaulting of values for specific fields on load of the Email/SendEmail Quick Action, but note that this cannot be used to remove the ToIds which are loaded based on the most recent email related to the Case. The ToIds will populate as outlined above regardless of the QuickActionDefaultsHandler execution.