diff --git a/chatmaild/pyproject.toml b/chatmaild/pyproject.toml
index 8fa212f8..3c312e74 100644
--- a/chatmaild/pyproject.toml
+++ b/chatmaild/pyproject.toml
@@ -71,5 +71,6 @@ commands =
 [testenv]
 deps = pytest 
        pdbpp 
+       pytest-localserver
 commands = pytest -v -rsXx {posargs}
 """
diff --git a/chatmaild/src/chatmaild/tests/test_filtermail_blackbox.py b/chatmaild/src/chatmaild/tests/test_filtermail_blackbox.py
new file mode 100644
index 00000000..08c71967
--- /dev/null
+++ b/chatmaild/src/chatmaild/tests/test_filtermail_blackbox.py
@@ -0,0 +1,78 @@
+import smtplib
+import subprocess
+import sys
+
+import pytest
+
+
+@pytest.fixture
+def smtpserver():
+    from pytest_localserver import smtp
+
+    server = smtp.Server("127.0.0.1")
+    server.start()
+    yield server
+    server.stop()
+
+
+@pytest.fixture
+def make_popen(request):
+    def popen(cmdargs, stdout=subprocess.PIPE, stderr=subprocess.PIPE, **kw):
+        p = subprocess.Popen(
+            cmdargs,
+            stdout=subprocess.PIPE,
+            stderr=subprocess.PIPE,
+        )
+
+        def fin():
+            p.terminate()
+            out, err = p.communicate()
+            print(out.decode("ascii"))
+            print(err.decode("ascii"), file=sys.stderr)
+
+        request.addfinalizer(fin)
+        return p
+
+    return popen
+
+
+@pytest.mark.parametrize("filtermail_mode", ["outgoing", "incoming"])
+def test_one_mail(
+    make_config, make_popen, smtpserver, maildata, filtermail_mode, monkeypatch
+):
+    monkeypatch.setenv("PYTHONUNBUFFERED", "1")
+    smtp_inject_port = 20025
+    if filtermail_mode == "outgoing":
+        settings = dict(
+            postfix_reinject_port=smtpserver.port,
+            filtermail_smtp_port=smtp_inject_port,
+        )
+    else:
+        settings = dict(
+            postfix_reinject_port_incoming=smtpserver.port,
+            filtermail_smtp_port_incoming=smtp_inject_port,
+        )
+
+    config = make_config("example.org", settings=settings)
+    path = str(config._inipath)
+
+    popen = make_popen(["filtermail", path, filtermail_mode])
+    line = popen.stderr.readline().strip()
+    if b"loop" not in line:
+        print(line.decode("ascii"), file=sys.stderr)
+        pytest.fail("starting filtermail failed")
+
+    addr = f"user1@{config.mail_domain}"
+    config.get_user(addr).set_password("l1k2j3l1k2j3l")
+
+    # send encrypted mail
+    data = str(maildata("encrypted.eml", from_addr=addr, to_addr=addr))
+    client = smtplib.SMTP("localhost", smtp_inject_port)
+    client.sendmail(addr, [addr], data)
+    assert len(smtpserver.outbox) == 1
+
+    # send un-encrypted mail that errors
+    data = str(maildata("fake-encrypted.eml", from_addr=addr, to_addr=addr))
+    with pytest.raises(smtplib.SMTPDataError) as e:
+        client.sendmail(addr, [addr], data)
+    assert e.value.smtp_code == 523