thunderbird and vpn

[#1]

I've installed a VPN on my windows box.  Now when I attempt to send or reply to mail in thunderbird I get a popup message
"<mail address> recipient rejected relay denied".  It works fine if I disable the VPN.

I've looked at various "howtos" but all have walkthroughs with menus which do not quite match what I see.

[#2]

assume you're using PN email?

if so you need to add authentication as with a VPN you're no longer on the PN network.