I would filter out at least whitespaces and comma. Aside from catching typos this also protects against header injection or spam abusenby sending to multiple recipients.

And yes, those should be defended when constructing the mail, but checking early doesn't cost much.

Nearly correct. You should also do a DNS lookup on $2 and check if it has either a MX record or an A record before trying to send it.

I do almost exactly that, but I replace the stars with plus. I don't think empty name or domain parts are legal?

Should be /^[^@]+@[^@]+$/ at least, because you need something before and after @

Breasts with a chest tattoo.