const Email = 'User@mail.ru';
var p: PChar;
begin
  p:=strpos(Email, '@');
  if p <> nil then inc(p);


function ParseEmail( Email: PChar): PChar;
//const Email = 'User@mail.ru';
var p: PChar;
begin
  p:=strpos(Email, '@');
  if p <> nil then inc(p);
  Result:=p;
end;
...
ShowMessage(ParseEmail('User@mail.ru'));