The error code provided and reviewing the logs do 100% indicate and confirm a VPN is being used. It will need to be disabled. This also includes services such as Cloudflare 1.1.1.1 or Cloudflare WARP
In the Hosting Control Panel:
E-mail Manager > E-mail Accounts > Click on the checkbox beside the mailbox you are looking to empty > Purge (this will clear all emails based on the folder and time period you specify)