If you have Thunderbird already set up using IMAP for your email account you can see the contents of all its folders and delete messages you no longer want to keep
First make sure deleted messages get moved to the Trash/Deleted folder:
Display the account details and click Copies & Folders. Ensure "Place a copy in" is ticked at the top with the "Sent" Folder radio button checked and the account name selected - or if that doesn't work select "Other" and set it there. Do the equivalent for Drafts further down the page.
Return to your folder contents lists. Sort the list in date or reverse date order, then select ranges of messages to be deleted and click the Delete button. Select File -> Empty Deleted from time to time to clear the Deleted folder, and select File -> Compact Folders to free up wasted space in Inbox..
David
