Fix recipients calculation

moved from https://gitlab.com/etke.cc/postmoogle/-/issues/67

---

```
sender@example.com ➡️ known@forwarding.com
cc: another@email.com
```

Reply is sent to (meta):

```
CC: another@email.com
RcpTo: postmoogle mailbox!
Recipients: another@email.com
To: known@forwarding.com
```