I use MS Outlook for my Mail. VPN was turned on (not by me) and send receive mail was blocked/blacklisted because of source IP address. Quite annoying. There must be spammers using the VPN to stop from getting blocked I would guess...
Many web sites do not allow access from known VPN IP addresses. They do this for copyright, legal, security, and political reasons. There is nothing a VPN provider can or should do to circumvent these legal protections.
Email providers block VPNs to protect your Account from hackers. When VPN is active, they see access attempt from a location they do not expect for your Account, so they either block or require additional security measures to sign you in.
If your Outlook is installed on your computer and not in the cloud, you can add the Outlook.exe to the Split Tunnel feature of Norton's VPN. Open the My Norton interface and click on the down arrow beside Turn on/off for the VPN feature, then click on Settings. Then click on Split Tunnel and then click the Add button. Navigate to your Outlook.exe file in C:/Program files or C:?Program files (x86).