Stuck Email not visible

I am trying to assist a customer who is using eMClient as his email client. He has tried to send an email with a badly formed address and eMClient throws up error messages that it cannot send or receive until the email with the incorrect address has been resolved. As a result he has a number of emails queued in the Outbox that will not send.

I have moved all queued emails to the Drafts folder after taking eMClient offline but unfortunately I cannot find the rouge email with the poor address.
Please can you suggest, if not in the outbox, where this email might be sitting as clearly the software is still trying to send it! I have even tried the webmail server ( to see if its sitting there but without success
Many thanks

The outbox is a local folder, so make sure they are visitable by going to menu/tools/settings/general/general and check “Show local folders”. You should be then able to delete the bad message.