I'm using a Service Console with cases. I have a Case Action (Quick Action) for sending an email. There is even a Predefined Field Value to set the case.customerID to the To: Address, but that doesn't seem to be happening. When I actually open this in the console view, to To: field remains blank.
I found the answer. The contact I was testing with has the same email as my salesforce login.
I don't remember where I found it, but if the email address that would be entered into the "To" field is an address that you can email FROM, then SF puts the email address in the BCC field, rather than the To field.
Solution: Test with different email addresses, not associated with your salesforce login.