'sent' mail folder disappears

Hi,

I recently created a new pop 3 mail account on my iPhone (apple mail app, 6S, IOS 11.3) and it works fine, except for the 'sent' box. When a message is initially sent it goes to the 'sent' folder and can be viewed. When the app is restarted, the 'sent' folder has vanished; leaving only the 'inbox'. The sent message also does not appear in the 'all sent' folder.

If I then send another email, the 'sent' box returns with all previous sent messages. Restart the app, and they've disappeared again.

Repeat ad infinitum.

My other 3 email accounts (all pop 3) work correctly. I have compared the settings of the new account to the old, and all the common settings are the same.

Any ideas?

Thanks.