I took the second expression from the first link, added separators and "case insensitive" flag, and set it into a custom identifier:
Code:
alias ismail {
  var %r = /^[A-Z0-9._%+-]+@[A-Z0-9.-]+\.[A-Z]{2,4}$/i
  if ($regex($1,%r)) { return $true }
  return $false
}

Now try
Code:
//echo -a $ismail(test) $ismail(@nothing) $ismail(123@456) $ismail(nick!ident@host.domain) $ismail(Test@something.com)


And use $ismail(<address>), like in
Code:
alias mailtest {
  if ($ismail($input(Enter eMail address:,eog,Mail Address))) { echo -a valid address given: $! }
  else { echo -a no/invalid address given }
}