[Geary] removing a message stuck on outgoing mail

[fede inventati org]

More than 2 weeks ago Geary could not save a sent email in Sent Mail 
folder, so it's still in the outgoing mail folder.
I cannot delete it.
Other messages have followed in that thread afterwards.

The message I see is: "This message was sent successfully, but could not 
be saved to Sent Mail".

How can I remove that message from the outgoing folder? Should I delete 
the database and restart?

Thanks
Federico