Returns perm error when there is `ptr:example.com` in the SPF

perm error is always returned for `@yahoo.com` emails because it uses `ptr`

```js
spf("test@yahoo.com", "74.6.143.26", "yahoo.com"); //> PermError
```

`yahoo.com`
```
v=spf1 redirect=_spf.mail.yahoo.com
```

`_spf.mail_yahoo.com`
```
v=spf1 ptr:yahoo.com ptr:yahoo.net ?all
```

