Using the Bitdefender VPN. Have no issues with any other application, except eM Client.
Unfortunately VPNs can interfere with many mail clients, and as @skybat advised need to be configured to allow the mail client to work.
So contact Bitdefender VPN, support to see how to configure it to allow eM Client access.
Bitdefender customer support site links below.
(Bit defender customer support page)
https://www.bitdefender.com/consumer/support
(Bit defender VPN support page)
https://www.bitdefender.com/consumer/support/get-help-vpn
Note: - There is also a user on this forum @ekalb that uses Bitdefender VPN and says it works ok. See what he advises in the thread below. So might give you some clues to help with that..