At present, when I move a message into a folder, the entire thread is moved - both received and sent messages.
I’m one of those people who organises all their email into folders, so it’s very important that the folders contain just what I expect. Almost all other mail clients (or, at least, almost all I’ve used, which is quite a few) only move received messages when moving, but Mailspring moves the entire thread. This means that my Sent folder no longer contains all my sent messages, but only those that haven’t been moved as part of a thread.
This is even more of a problem when I delete a thread (i.e. move it to Trash): I don’t want my sent messages deleted, just the received ones.
This all means that Mailspring doesn’t play very nicely with other mail clients that don’t behave this way. (For example, I use Maildroid on my phone, which is a more traditional non-threaded client, and it very much expects the Sent folder to hold all the sent emails.)
An option (to preserve existing behaviour for users who’re accustomed to it) to apply Move and Copy actions only to messages that have been received and not those that have been sent from the current account. This should still allow the threaded display to work, in a similar way to the existing default hiding of messages in the thread that have been moved to Trash.
This is similar to some other requests (e.g. this one) but is different IMO. It’s also similar to this thread relating to some bug reports - although, again, I’m not sure this is a bug but more “working as designed but not ideal”!