It's helpful to provide information when creating any question. The info you supplied is identical to the question. What OS are you using ? What version of Thunderbird are you using ? Are you talking about an IMAP Inbox or a Pop Inbox OR are you using 'Unified Folders' view option ? In which case are you talking about dragging from the 'unified Inbox' itself and not the actual Inbox of an account ? Upload an image which shows the Inbox and the subfolder - is it a subfolder of the account or of the Inbox ? Are you able to left click hold down on email in Inbox and drag email to hover over the subfolder - at this point what do you see - do you see the red circle with red line through it OR do you get the small outline of rectangle icon indicating you should be able to drop into that folder. Does the subfolder ''Name'' show using ''italic'' font ? if yes, then it cannot store emails, but it can contain folders that have emails.