My apologies because I’m nearly certain this would have been asked, but I haven’t found it by search.
When I reply to an email left in my inbox, the reply can only be found in the sent folder. Is there a setting I’ve missed that would allow replies to be shown in the original inbox message?
That doesn’t even make any sense. Why would a sent message apoear in your inbox? More importantly, why would you want a sent message in your inbox?
That sounds suspisciously like the sort of inexplicable behavior that Gmail does. In which case, if you want a mail client that behaves like Gmail, why not just use Gmail?
Its suspicious to have your reply appear in the message, like a thread? And I don’t use gmail because I don’t use any google product.
By that account, Thunderbird, Protonmail, virtually anything out there that supports a thread-like history is suspicious. Is that right?
My personal solution to this issue in both K-9 and other mail clients is to add my own email address to the automatic BCC list of all outgoing messages. This can be found under Account Settings → Sending Mail → Composition Defaults.
Perfect, thank you very much!
If you add yourself as a BCC recipient, you should disable uploading sent messages under Settings → [Account] → Sending mail → Upload sent messages. Otherwise you’ll end up with two copies of every message sent using K-9 Mail.
Alternatively, you could select the Inbox under Settings → [Accout] → Folders → Sent folder. Then sent messages will be uploaded to the Inbox.
Let me expand this a bit by saying: Every decent email client should be capable of putting sent messages in the same folder as the mail I replied to or, in the case of “pristine” sent mail, in the INBOX.
The reason for this spells CONTEXT and THREADING.
I have different folders for different purposes. Some mails remain in the inbox because I am too lazy or it does not fit the purpose of a folder. Regardless, having sent emails in the SAME folder as the rest of the “same purpose emails” makes perfekt sense to me, because all emails pertaining to a certain purpose are in the same folder.
Apparently this is not what you want, @linkp, and that is fine. Just don’t say it does not make sense. Because it does. To me and a lot of others. 
Have a nice day!
Yes, for me it also makes sense to have all messages (inbox and my responses) in one view. But not sure to have a own copy of that response in inbox as well.
Maybe just an option to have “joined view” on both inbox and sent? If it is a view only then at any point you could switch on/off ( also messages sent in past would work) and no duplicates are kept.