LOVE the Tag feature. Wish a Tag could be added to a new message, and then all replies, etc to that message would hold that Tag.
I use Filters so incoming messages get Tagged according to what group/project I'm working with, but my replies to them, and new messages to them, do not get tagged, The SENT folder has no tags in it unless I've manually selected and added them.
Opaite Mbohovái (1)
Why can't you apply a filter that acts on the Sent folder, with the same criteria you apply to incoming messages, e.g. Subject contains xxxxxx, Tag Message as yyyy. In the Tools/Message Filters window there is a selection box: Run selected filter(s) on:. You might want to edit the Subject of incoming mail (with an add-on) to make this easier to work. There's also an add-on to manage a collection of Subjects for replies and new messages.