Email has certain well-known and long-established standards about how to respond to an incoming message. You are working with a pretty crap system if it hasn't been configured to work with these standards. If you want a response to be logically connected to the original message, it needs to quote back-references to the original message. Reply and Forward are the two methods offered to do this. And the standards encourage, if not actually mandate, the use of Re: and Fwd: (or equivalent) to alert a recipient to the fact that the message is connected to an earlier message. The tool offered by Thunderbird to send an old message in a new context is "Edit as New", where a new message is generated and populated with the first message's contents, but without the Reply or Forward chaining. Using this would mean that your reply to acknowledge the ticket would have none of the usual internal references to the incoming message. But Edit As New isn't offered as a filter action. You can change the "Fwd:" to something else by modifying this preference: mail.forward_subject_prefix I haven't tried it with an empty string. But this appears to open the door to a filter that forwards the message without a "Fwd:" on the subject line.