By enabling VPN, you always establish a point-to-point connection (tunnel) between your device and a VPN server.
With active VPN, your device establishes an encrypted connection with the VPN server, which in turn forwards all data to the website servers. As a result, your device is excluded from the local network during this time and you cannot connect to the network printer/network devices or an e-mail server. This prevents local email clients like Outlook, Thunderbird, etc. from sending or receiving email.
Only after disabling VPN, you will be re-connected to your local network as usual and can use network printers/network devices again or send and receive e-mails.