How to respond to email from Hawaii
If you are sincerely happy that your correspondent is enjoying an islands vacation: You can say something nice.
If you are tired of winter and don't need to be reminded that it's not like this everywhere: Adopting a tone of mild sarcasm might be the way to go.
If you are pretty much an ignoramus when it comes to U.S. geography: You can write back and ask if the email sender is worried about being allowed back in the country.