diff --git a/flake.nix b/flake.nix
index adf9d65..3620b7c 100644
--- a/flake.nix
+++ b/flake.nix
@@ -46,6 +46,7 @@
               isort.settings.flags = "--profile black";
               pylint.enable = false; # doesn't work correctly with module imports in flake
               detect-private-keys.enable = true;
+              detect-private-keys.excludes = [ "tests/" ];
             };
           };
           app = self.packages.${system}.smtprd-ng;
diff --git a/smtprd_ng/smtprd.py b/smtprd_ng/smtprd.py
index 074ea46..f712365 100644
--- a/smtprd_ng/smtprd.py
+++ b/smtprd_ng/smtprd.py
@@ -400,11 +400,16 @@ class SMTPServer(Controller):
         return True
 
 
-def main() -> int:
-    """Main routine
+def parse_args(args=None) -> argparse.ArgumentParser.parse_args:
+    """Parse arguments
 
-    Returns:
-        int: exit code
+    Parameters
+    ----------
+    args : List of strings
+
+    Returns
+    -------
+    argparse.ArgumentParser.parse_args : Namespace of arguments
     """
     parser = argparse.ArgumentParser(
         description=__doc__.strip(),
@@ -417,9 +422,21 @@ def main() -> int:
         default="./config.ini",
         help="configuration file",
     )
-    args = parser.parse_args()
-    setlocale(LC_ALL, "C")  # for strftime
+    return parser.parse_args(args)
 
+
+def main(args=None) -> int:
+    """Main routine
+
+    Returns:
+        int: exit code
+    """
+    args = parse_args(args)
+    setlocale(LC_ALL, "C")  # for strftime
+    if len(args.config) == 0:
+        raise OSError("No config file supplied")
+    if not Path(args.config).is_file():
+        raise OSError("Config file not found: " + str(args.config))
     try:
         config: Config = Config.from_ini(args.config)
     except RuntimeError as e:
diff --git a/tests/test_smtprd_ng.py b/tests/test_smtprd_ng.py
index 2f35d34..c0004ca 100644
--- a/tests/test_smtprd_ng.py
+++ b/tests/test_smtprd_ng.py
@@ -19,12 +19,14 @@
 Tests for smtprd_ng
 """
 # pylint: disable=protected-access
+# pylint: disable=unused-argument
 
 import configparser
 import email
 import email.message
 from pathlib import Path
 
+import pytest
 from M2Crypto import BIO, SMIME, X509
 
 from smtprd_ng import smtprd
@@ -134,3 +136,21 @@ def test_client_sign():
     p7, data = SMIME.smime_load_pkcs7_bio(buf)
     v = s.verify(p7, data)
     assert "Test to sign" in v.decode()
+
+
+def test_cli_no_config_file(capsys):
+    """Test whether config file parameter is supplied"""
+    test_args = ["--config", ""]
+    with pytest.raises(Exception) as e_info:
+        smtprd.main(test_args)
+    assert e_info.typename == "OSError"
+    assert str(e_info.value) == "No config file supplied"
+
+
+def test_cli_config_file_not_found(capsys):
+    """Test whether config file is found"""
+    test_args = ["--config", "doesnotexist.conf"]
+    with pytest.raises(Exception) as e_info:
+        smtprd.main(test_args)
+    assert e_info.typename == "OSError"
+    assert str(e_info.value) == "Config file not found: doesnotexist.conf"