This link specifies the Outbox but it works for the other mail boxes as well and may work for Deleted Items click here
You may still have to delete the stuck message after applying the fix. Remember where the fix is as you may need it again.
Huawei P10 review
1995-2015: How technology has changed the world in 20 years
How VR is being used to simulate space
New iPad, iPhone SE & Red iPhone 7 on sale now
Synology invites you to visit them at Insomnia on 26th – 29th August. Book your tickets now and visit Synology.