153.0.1 Error: current command did not succeed. The email server responded: APPEND invalid flag list", because IMAP server is rejecting the APPEND command because Thunderbird is sending two junk-related flags simultaneously: $notjunk and NonJunk

Here is some information supplied by the email account provider:

When creating a new folder, the server shows error: "{imaps} Bad flag in APPEND command: Nonjunk"

The error is clear — the IMAP server is rejecting the APPEND command because Thunderbird is sending two junk-related flags simultaneously:

$notjunk — a valid IMAP keyword flag NonJunk — a non-standard flag (no backslash prefix) that the server doesn't accept

This is a Thunderbird-side behavior — it's sending both $notjunk and NonJunk together when moving messages to newly created folders, and the server's IMAP parser rejects NonJunk as invalid.

> With Thunderbird 153.0.1, after creating a new subfolder in the account, when attempting to move emails to the new folder, I receive the following error:

> "Thunderbird: The current command did not succeed. The email server responded: APPEND invalid flag list"

Where are you moving the messages from? Is it the same account where you created the new subfolder or a different account or maybe Local Folders. I ask this because imap APPEND is only used when you are moving or copying messages from another account into the an imap account. If you are copying or moving message inside the same account, APPEND is not used.

I have a folder in my main imap account containing two messages, each having both $NotJunk and NonJunk flags. I then copy the messages to a folder in a different imap account and the APPEND commands do not contain $NotJunk or NonJunk. So it appears that these flags/tags/keywords are getting stripped off before the APPEND occurs.

So please clarify exactly how you are doing the message moves. Also, it would be helpful to know exactly what your providers imap server type is or at least who the provider is.