Sometimes the best way to entice new visitors and subscribers is with word-of-mouth endorsements of your premium content, but obviously new visitors will want to sample that content.
The Wall Street Journal came up with a smart way to avoid paywall corruption while still letting new visitors sample recommended article. They use the subject line to start with their brand and then the headline of the story. So it’s not generic and probably far more likely to get opened. The friend who sent the link is identified in the “From” and in the actual text of the message itself, which helps this avoid the appearance of spam. Then, the message notes how long you have to examine the content before it goes back behind the paywall. That helps maintain the premium value of the content.
Just don’t use that email for marketing purposes (the referred friend hasn’t opted-in to anything). But you can upsell them in the same email, if you do it tastefully!