I've been researching online to verify this, and then tested this morning, and I'm confused as to why it works this way. Could someone please verify if I'm just missing some extra step?
An email sent via an email-to-case automated reply to the Client, with a ThreadID on the subject line, allows the Client to REPLY to the email and that reply ends up on the Case in Salesforce. Good.
An email sent (workflow or otherwise?) from the Case, with the ThreadID on the subject line, does NOT allow the Client to reply and have that comment return to the case. Bad.
Am I just missing something extra I need to do?? :-( Why have ThreadID and ONLY allow it to work with email-to-case emails??
EDIT: An online reference to this issue.
EDIT 2: I've half answered the question through more research. The thread_id works FINE for getting the message back to SF if the FROM field is properly set. If the FROM field is someone's personal email, the reply never GOES BACK to SF. If, however, the FROM field is set to either the long-address for SF email-to-case, or more easily to the email that is you have FORWARDING to that long address (firstname.lastname@example.org let's say), then SF gets the email, sees the Thread on the Subject, and sends it to the Case it belongs to.
THE QUESTION THEN BECOMES (still reearching): Can we make the "default" FROM address on each Case Record Type be a specific email address?
The closest I see to this is in the Email Alerts area, ("Make this address the default From email address for this object's email alerts") but that affects all email alerts based on waht I know.. not "which FROM address" would be used if they click Send Email in a Case.