[n5bb]

You are changing the stated issue.

• Your original post says that the message was filed into the Trash folder by a rule. So if you later try to manually undelete that message, where would you want it to go? The message was never deleted, so I have a hard time understanding why undelete should have any effect.
• If a message is already in the Trash folder, you can't perform a regular Delete operation on it, but you can Delete Permanently the message. So the Undelete action can't possibly have an effect on messages in Trash which were filed by rules into the Trash folder.
• You can't currently create a subfolder of Spam or Trash using the web interface, since those folders are greyed out and unselectable in the folder create tool. I think that's because of side effects. I think that we used to be able to create subfolders of those folders, but this is no longer possible using the web interface. You can create such subfolders using an email client - I just did this using Thunderbird. But that doesn't mean that it's supported with the other Fastmail features.
• An interesting aside is that if you create the subfolder Trash.foo using an email client, you can then create the sub-subfolder Trash.foo.doo using the web interface. So the web interface assumes you are only creating folders using that interface, and preventing you from creating subfolders of Trash and Spam is therefore sufficient from creating any subfolder structure under those folders.
• So if I were you I would not use subfolders of Trash or Spam (even if create in an email client) if you expect to use the web interface.

Bill