feat: add pneumatube gateway bridging smolmail and SMTP via LMTP

tests/test_smtp_client.py

@@ -0,0 +1,18 @@
+import pytest
+
+from pneumatube.smtp_client import InvalidAddress, validate
+
+
+def test_validate_accepts_plain_address():
+    assert validate(" bob@example.org ") == "bob@example.org"
+
+
+@pytest.mark.parametrize("address", [
+    "not-an-email",
+    "bob@localhost",
+    "bob@example.org\r\nBcc: evil@example.org",
+    "bob@example.org\nX-Injected: 1",
+])
+def test_validate_rejects_bad_addresses(address):
+    with pytest.raises(InvalidAddress):
+        validate(address)