Describe the bug

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

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:

   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.