If you have Windows, run the following command below as per @Gary post from the following thread.
Gary Leader
An e-mail in Trash that won’t delete - Mail - eM Client
Close eM Client and open a Windows Command Prompt. Not a powershell, but a command prompt.
Paste in this command:
“C:\Program Files (x86)\eM Client\MailClient.exe” /dbrepair
Restart eM Client. Any undeletable messages should be removed.