As @gg30340 has alluded to, you need to be very careful here or you run the list of losing the email in Outlook. Just out of curiosity, is this local Outlook as part of Office or the online Outlook? Office Outllook defaults to a local copy only when using POP3. That means your emails are downloaded only onto the computer running Outlook & then deleted from the server. This means that they only exist within that local copy of Outlook. So the first thing I'd do is find the setting within your BT account on Outlook to keep a copy on the server. That won't send old stuff back to the server but will prevent any more from being removed.
As you're using K9 on the phone, maybe consider Mozilla Thunderbird on the PC, as the K9 app has been integrated under the Thunderbird project. If you were to set Thunderbird up as IMAP then it should match K9 on the phone & you can just use Outlook as an archive for old stuff.
I don't use K9 myself but did migrate to Thunderbird from Outlook a couple of years ago. Like all changes it takes a bit of getting used to but not too painful. Hopefully the K9 tie-up will make it more intuitive for you.
