In the IMAP standard, when you "delete" an item, it isn't actually deleted permanently. Instead, the message marked as "deleted." This is often displayed in email clients as a crossed-out entry in your mailbox. At this stage, the email still exists in your mailbox, but it has now been flagged for deletion. To actually delete the item permanently, you have to "purge" the mailbox. Most IMAP clients have an option in one of the menus or on the toolbar called "purge," which permanently deletes the email.
Most people find this system counterintuitive, so almost all email clients these days don't "mark messages as deleted," but instead just move the email to your Trash folder. When you log into our web interface, we will automatically purge any emails marked for deletion, so you will never see a "crossed-out" email. If you see this in an email client, there might be a setting to change the behavior, otherwise you will need to update to a newer app, or use our web interface instead.
Deleting folders in Thunderbird
Fastmail's IMAP server doesn't allow subfolders of Trash or Spam. This an intentional decision, but can cause issues when trying to delete a folder in Thunderbird.
Thunderbird folder deletion works similar to email deletion. It moves a deleted folder to be a subfolder of the Trash folder. Since our server doesn't allow that, it throws back a "Permission denied" error indicating that Thunderbird is attempting to delete the folder using a mechanism that Fastmail's IMAP server doesn't support.
You can switch Thunderbird's email deletion setting to "Delete immediately." But this also means Thunderbird will immediately delete emails as well, so it is recommended that you switch back to the "Move to trash" option after the folder is deleted.
To adjust this setting in Thunderbird, go to the "Account Settings → Fastmail account → Server Settings" screen in Thunderbird. Under the "When I delete a message" section, choose "Remove it immediately." Enabling this setting will permanently remove emails and folders when you delete them.
Alternatively, you can use Fastmail's web UI to directly delete folders.