If you're using your ISP's mail service, they may also have web-based access, so you can get into your email with a web browser. Try doing that and see if you can then delete the message which is causing the problem.
Surface Pro (2017) vs Surface Pro 4
20 groundbreaking 3D animation techniques
How to mine Bitcoin on Mac
Synology invites you to visit them at Insomnia on 26th – 29th August. Book your tickets now and visit Synology.