fix issue, inserting 'from' from envelope into message headers.

add test case to expect failure when there really is no 'from'

lettre_email/src/lib.rs

@@ -430,6 +430,10 @@ impl EmailBuilder {
         if !self.from.is_empty() {
             self.message = self.message
                 .header(Header::new_with_value("From".into(), self.from).unwrap());
+        } else if let Some(from) = envelope.from() {
+            let from = vec![Address::new_mailbox(from.to_string())];
+            self.message = self.message
+                .header(Header::new_with_value("From".into(), from).unwrap());
         } else {
             Err(EmailError::Envelope {
                 error: LettreError::MissingFrom,

lettre_email/tests/build_with_envelope.rs

@@ -15,4 +15,18 @@ fn build_with_envelope_test() {
         .text("message")
         .build()
         .unwrap();
+}
+
+#[test]
+fn build_with_envelope_without_from_test() {
+    let e = Envelope::new(
+        None,
+        vec![EmailAddress::new("to@example.org".to_string()).unwrap()],
+    ).unwrap();
+    let _email = EmailBuilder::new()
+        .envelope(e)
+        .subject("subject")
+        .text("message")
+        .build()
+        .unwrap_err();
 }