Since IMAP stores email messages on the server, they will be available on all email clients you use. On K-9, usually, you won’t need to configure your IMAP folders except the “Sent” folder (although you can configure other folders, too), so you can have the same folder for all your email clients.
To set which folder you want to use as your “Sent” folder:
- Make sure the folder is already created. If it isn’t, you can log into your email account via http://webmail.yourdomain.com then create it there
- Open K-9 mail, and open up the Menu
- Tap More → Settings → Account Settings → Folders → Folder for Sent Objects
- Select the IMAP folder you wish to use to store email messages sent from your device
If you need to, you can repeat this process for your “Drafts”, “Trash” and other special folders, in the “Folders” menu item.