Even writing a simple email could be tricky during this pandemic. Your recipient(s) are on the edge -- everyone is -- and there's no etiquette rulesbook out there that guarantees actions that will keep you in good standing with your customers and relationships. At least there's an infographic that ranks the best (and worst) email sign-off. It's a start (sort of.)
Here's a quick note to front-end programmers: here's one way to validate an email address using JavaScript, without relying on shady regex.