CREATE failed: the destination rejects the folder name

a5 NO Mailbox does not exist, or must be subscribed to.

The destination server refused to create a folder with that name. Usually the folder is fine — the hierarchy separator is not.

• Different separators: Dovecot uses a slash, legacy Courier uses a dot, so INBOX.Sent becomes a top-level folder with a dot in its name instead of a child folder.
• Forbidden characters in the name — on some servers that is a dot, slash or backslash.
• Cyrillic names in modified UTF-7 mishandled by a third-party tool.
• A path too long or nesting too deep for the destination server.

1. Enable automatic folder mapping — the separator is detected on both sides and paths are rewritten.
2. Set an explicit rename rule if the structure is unusual.
3. Check what the server reports: LIST "" "*" shows the separator in its reply.
4. Map special folders by purpose, not by name: Junk and Spam are the same folder.

Test the connection in MoveMailbox