If messages won’t clear from your eM Client trash folder and “are not showing in your webmail online”, then suggest you close eM Client and run the repair command which normally fixes that.
Note: Before running the repair, make a manual backup via “Menu / Backup” incase you need to restore for any reason. You can see when the backup is complete in Show Operations via the dropdown on the right of Refresh.
To repair eM Client for Windows see @Gary link below.
https://forum.emclient.com/t/email-message-that-cannot-be-deleted/86994/2
To repair eM Client for Mac see @Gary link below.
https://forum.emclient.com/t/email-message-that-cannot-be-deleted/86994/6