Hi there,
this is driving me crazy and it’s getting worse. 
For a long time all folders but Inbox and Trash did only sync when I entered them and pulled down to do a manual reload (I had no the energy to look into this, as I still was able to get the nails(
Since the update to 12.1 (could be that I skipped 12.0) it does not work at all anymore.
Example:
- I start typing a mail in Thunderbird on my computer and save it in drafts
- I found no way to get this mail in K9
- Same with sent mails
- It also does not work the other way around (mails from the sent and drafts folder in K9 don’t show up anywhere else)
- The Mails are definitely on the server as I can see them via webmail there
- The mailbox is a generic IMAP at df.eu
What have I tried:
- I checked folder names for a German/English problem (there is just one folder “sent in the account, nothing like “gesendet” in the refreshed folder list).
- Strangely there a two folders with “Drafts” in the name. One is only “Drafts” and the other is “INBOX.Drafts” - but this is also the case for the Trash folder where everything works fine and this could not explain the problem with the sent (and archives) folder.
- Drafts and INBOX.Drafts as well as Trash and INBOX.Trash instantly have the same content in Thunderbird, as if they are symbolic links on the server.
- In K9 the INBOX.Drafts and INBOX.Trash are empty and won’t load anything on a refresh
- In K9 Drafts does not sync with the server (despite it’s active in the folder settings) and Trash works just fine.
- I checked that the upload for sent mails is activated
- I enabled sync for every folder found in the refreshed folder list but couldn’t find the mail saved in Thunderbird in any of them
- I enabled everything available under folder settings, except notifications and integration to joint inbox (I don’t use that)
- Sync timeframe is indefinitely and limit is set to 1000 messages
- “Load next 1000 messages” does nothing
- I even recreated the mail account in K9
I know this screams: “This is a folder naming issue” and it’s seems there could be no other reason, but I looked into that in my tries #1, #2 and with no result.
Just as an overview here the stucture and behaviour in both apps:
Summary K9
- INBOX (syncs fine)
- INBOX.Drafts (is empty and does not sync)
- INBOX.Trash (is empty and does not sync)
- Drafts (stores local drafts but does not sync)
- Sent (stores local sent mails but does not sync despite upload and sync are active in settings)
- Trash (syncs fine)
Summary Thunderbird
- INBOX (syncs with webmail)
- INBOX.Drafts (instantly same content as Drafts)
- INBOX.Trash (instantly same content as Trash)
- Drafts (syncs with webmail)
- Sent (syncs with webmail)
- Trash (syncs with webmail)
INBOX.Drafts and INBOX.Trash are not deletable.
I have no ideas left what I can look into… 
Looking curiously forward to your suggestions,
Thomas
Ps: I don’t know if this is related: the “search on server” does also not work (I would rather open another topic for that to keep things clean and separate)