Replace all instances of “mail” with “email”

In the Tools > Test Email page, the title reads “Test Mail” as well as “mail” in multiple other places. Mail is the physical transport of letters and packages, which is why its digital counterpart was named e-mail (or email) to signify that it is electronic mail: the electronic version of physical mail. Therefore, please update all the instances of “mail” to “e-mail” or “email”.