Not getting my mail when VPN is on!

Turn VPN Off & you don't need Norton.

Check the Wi-Fi connection, see the following Apple articles:

Connect to Wi-Fi on your iPhone, iPad, or iPod touch

Connect to Wi-Fi on your iPhone, iPad, or iPod touch - Apple Support

See this Apple article: If your iPhone, iPad, or iPod touch won’t connect to a Wi-Fi network

If your iPhone, iPad, or iPod touch won't connect to a Wi-Fi network - Apple Support

Not enough information in your post to give any chance of addressing the issue.

For a start, which mail? Are you using iCloud mail? Gmail? your own corporate?

Assumedly you can when the VPN is off. Is there a difference whether Mail is open or closed when the VPN connection is established?

What VPN are you using? Is it your corporate VPN? some 'free' VPN service?

What is the experience? What error messages do you get?

Answers to these (and likely more) will have a greater chance of pointing you towards a solution.