Thank you. This worked. I do have an issue that only the last two years (approx) were imported from my Apple Mail iCloud account and on further investigation all older sent messages have disappeared from Apple Mail account. I had simply experted the sent folder from Mail and imported to Postbox. Interestingly, I was checking out BlueMail last week and all the folders, incl Sent folder, appeared instantly there without having to do any exporting, importing or 'subscribing'.