Add RFC email headers to report messages

tests/test_email_report.py

@@ -1,7 +1,11 @@
 from pathlib import Path
 
 from pve_backup_report.config import EmailConfig
-from pve_backup_report.email_report import build_report_message, send_report_email
+from pve_backup_report.email_report import (
+    build_report_message,
+    message_id_domain,
+    send_report_email,
+)
 
 
 def test_build_report_message_attaches_pdf(tmp_path: Path) -> None:
@@ -20,6 +24,9 @@ def test_build_report_message_attaches_pdf(tmp_path: Path) -> None:
     assert message["Subject"] == "Rapport PVE"
     assert message["From"] == "report@example.invalid"
     assert message["To"] == "admin@example.invalid"
+    assert message["Date"]
+    assert message["Message-ID"].endswith("@example.invalid>")
+    assert message["Auto-Submitted"] == "auto-generated"
     attachments = list(message.iter_attachments())
     assert len(attachments) == 1
     assert attachments[0].get_filename() == "rapport.pdf"
@@ -27,6 +34,13 @@ def test_build_report_message_attaches_pdf(tmp_path: Path) -> None:
     assert attachments[0].get_payload(decode=True) == b"%PDF-1.7"
 
 
+def test_message_id_domain_uses_sender_domain() -> None:
+    assert message_id_domain("PVE Report <pve@example.invalid>") == "example.invalid"
+    assert message_id_domain("pve@example.invalid") == "example.invalid"
+    assert message_id_domain("invalid") is None
+    assert message_id_domain(None) is None
+
+
 def test_send_report_email_uses_starttls_and_auth(tmp_path: Path, monkeypatch) -> None:
     pdf_path = tmp_path / "rapport.pdf"
     pdf_path.write_bytes(b"%PDF-1.7")