SPF not checked for incoming email (missing Envelope-From)

## Describe the bug
When an **incoming email** is downloaded from the Postal web panel and then analyzed locally with SpamAssassin using:

```bash
spamassassin -t -D spf < email.eml
```

The SPF plugin does not evaluate the message properly and logs the following debug message:

```
Nov 8 10:57:12.553 [1244] dbg: spf: checking to see if the message has a Received-SPF header that we can use
Nov 8 10:57:12.573 [1244] dbg: spf: using Mail::SPF for SPF checks
Nov 8 10:57:12.573 [1244] dbg: spf: cannot get Envelope-From, cannot use SPF
```

This shows that SpamAssassin is unable to get the **Envelope‑From (Return‑Path)** from the downloaded `.eml` file and as a result the SPF check is skipped.

---

## To Reproduce
1. Receive an incoming email in **Postal**
2. Open the Postal web panel
3. Download the raw message (`.eml` file)
4. Run the command:
```bash
spamassassin -t -D spf < email.eml
```
5. Observe the SPF debug output

---

## Expected behaviour
SpamAssassin should be able to extract or infer the Envelope‑From of the message (e.g. from the `Return‑Path:` header) and perform SPF verification successfully, even when the message is analyzed manually after download.