Phantom unread message in Mail.app Trash, Tiger

anyone have old school mac os x knowledge me? grandfather has g4 imac still running os x 10.4 tiger, , pop email account in mail.app showing 1 unread message in empty trash folder. pop account that's no longer primary still checked. don't have option rebuild trash folder, though other account's trash folder does. shows arrow "unfold" it, there no other folders within. ideas on how might solve this?

 

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