To help in communications, it will help to see a screenshot of your copies&folders pane for the account

How do I do that without showing my account name?

See attached example.

Hope you can help me now.

Very puzzling. I suggest verifying this by - highlighting sent folder in folder pane, rightclick and select properties. That should show the actual setting. - also, look in profile folder. the imapmail\<fastmail> folder and the Mail\Local Folders folder to see if the messages are potentially in both places. But, honestly, I've never seen anything remotely like this.

Thanks for trying to help. Unfortunately this is too complicated for me to fix . So after 20 years of using it, and it being a 1st rate email client,I'll have to use webmail instead.

Being a stubborn and inquisitive so so I've been looking more closely at where messages are going. Up to 27th April sent messages were going into a Fastmail sent items folder. Since then Local folders -sent. Drafts that used to go in Fastmail- drafts are going into Local folders- drafts. Local folders- drafts has 4 very small test emails in it, and 3.1 MB size on disk.

For each of the three folders, you are using an "other" option. What happens when you select the first option?

I use Fastmail. The first option works fine for me.

. It's a recently occurring problem. First time I noticed was after getting an email out of the blue re 'trying to use a 3rd party ap' . That was on the 26th April. Re using the 1st options as you suggested. It didn't work.

do you get a message about some sort of error and suggesting saving in Local folders > something?

When you right click on the folder Fastmail in your folder pane and select subscribe. Does it show drafts and sent as subscribed? (if the right click menu does not show subscribe. Please confirm the account is configured as IMAP by going to the account settings and clicking on server settings. Is the Server type shown, first thing before server name, IMAP or POP?

Thank you for replying. No message about an error. Drafts and sent are subscribed. I have a sent items folder I prefer to use than sent. Up until recently sent posts were going into there. In fact all was going well re posts going where I wanted them to go. I'm no person's idea of a tech genius, but I can get the basic things right. IMAP is used, but I had old server name for it. I changed that, but it has made no difference as to which folders draft and sent messages are being filtered into.