Validate email address

Problem/Question/Abstract:

How to validate an email address

Answer:

function IsValidEmail(const Value: string): boolean;
  function CheckAllowed(const s: string): boolean;
  var
    i: integer;
  begin
    Result := false;
    for i := 1 to Length(s) do
    begin
      // illegal char in s -> no valid address
      if not (s[i] in ['a'..'z', 'A'..'Z', '0'..'9', '_', '-', '.']) then
        Exit;
    end;
    Result := true;
  end;
var
  i: integer;
  namePart, serverPart: string;
begin // of IsValidEmail
  Result := false;
  i := Pos('@', Value);
  if (i = 0) or (pos('..', Value) > 0) then
    Exit;
  namePart := Copy(Value, 1, i - 1);
  serverPart := Copy(Value, i + 1, Length(Value));
  if (Length(namePart) = 0) // @ or name missing
  or ((Length(serverPart) < 4)) {// name or server missing or } then
    Exit; // too short
  i := Pos('.', serverPart);
  // must have dot and at least 3 places from end
  if (i < 2) or (i > (Length(serverPart) - 2)) then
    Exit;
  Result := CheckAllowed(namePart) and CheckAllowed(serverPart);
end;