diff --git a/backend/app/services/blocklist_service.py b/backend/app/services/blocklist_service.py
index e226fc6..296992a 100644
--- a/backend/app/services/blocklist_service.py
+++ b/backend/app/services/blocklist_service.py
@@ -304,6 +304,16 @@ async def import_source(
         try:
             await jail_service.ban_ip(socket_path, BLOCKLIST_JAIL, stripped)
             imported += 1
+        except jail_service.JailNotFoundError as exc:
+            # The target jail does not exist in fail2ban — there is no point
+            # continuing because every subsequent ban would also fail.
+            ban_error = str(exc)
+            log.warning(
+                "blocklist_jail_not_found",
+                jail=BLOCKLIST_JAIL,
+                error=str(exc),
+            )
+            break
         except Exception as exc:
             skipped += 1
             if ban_error is None:
diff --git a/backend/app/services/jail_service.py b/backend/app/services/jail_service.py
index c33eb5c..8990215 100644
--- a/backend/app/services/jail_service.py
+++ b/backend/app/services/jail_service.py
@@ -131,6 +131,10 @@ def _ensure_list(value: Any) -> list[str]:
 def _is_not_found_error(exc: Exception) -> bool:
     """Return ``True`` if *exc* indicates a jail does not exist.
 
+    Checks both space-separated (``"unknown jail"``) and concatenated
+    (``"unknownjail"``) forms because fail2ban serialises
+    ``UnknownJailException`` without a space when pickled.
+
     Args:
         exc: The exception to inspect.
 
@@ -142,6 +146,7 @@ def _is_not_found_error(exc: Exception) -> bool:
         phrase in msg
         for phrase in (
             "unknown jail",
+            "unknownjail",  # covers UnknownJailException serialised by fail2ban
             "no jail",
             "does not exist",
             "not found",
diff --git a/backend/tests/test_services/test_blocklist_service.py b/backend/tests/test_services/test_blocklist_service.py
index 9675f71..633fcfe 100644
--- a/backend/tests/test_services/test_blocklist_service.py
+++ b/backend/tests/test_services/test_blocklist_service.py
@@ -187,6 +187,33 @@ class TestImport:
         assert result.ips_imported == 0
         assert result.error is not None
 
+    async def test_import_source_aborts_on_jail_not_found(self, db: aiosqlite.Connection) -> None:
+        """import_source aborts immediately and records an error when the target jail
+        does not exist in fail2ban instead of silently skipping every IP."""
+        from app.services.jail_service import JailNotFoundError
+
+        content = "\n".join(f"1.2.3.{i}" for i in range(100))
+        session = _make_session(content)
+        source = await blocklist_service.create_source(db, "Missing Jail", "https://mj.test/")
+
+        call_count = 0
+
+        async def _raise_jail_not_found(socket_path: str, jail: str, ip: str) -> None:
+            nonlocal call_count
+            call_count += 1
+            raise JailNotFoundError(jail)
+
+        with patch("app.services.jail_service.ban_ip", side_effect=_raise_jail_not_found):
+            result = await blocklist_service.import_source(
+                source, session, "/tmp/fake.sock", db
+            )
+
+        # Must abort after the first JailNotFoundError — only one ban attempt.
+        assert call_count == 1
+        assert result.ips_imported == 0
+        assert result.error is not None
+        assert "not found" in result.error.lower() or "blocklist-import" in result.error
+
     async def test_import_all_runs_all_enabled(self, db: aiosqlite.Connection) -> None:
         """import_all aggregates results across all enabled sources."""
         await blocklist_service.create_source(db, "S1", "https://s1.test/")
diff --git a/backend/tests/test_services/test_jail_service.py b/backend/tests/test_services/test_jail_service.py
index 1430abd..6f3f902 100644
--- a/backend/tests/test_services/test_jail_service.py
+++ b/backend/tests/test_services/test_jail_service.py
@@ -323,6 +323,20 @@ class TestBanUnban:
         with pytest.raises(ValueError, match="Invalid IP"):
             await jail_service.ban_ip(_SOCKET, "sshd", "not-an-ip")
 
+    async def test_ban_ip_unknown_jail_exception_raises_jail_not_found(self) -> None:
+        """ban_ip raises JailNotFoundError when fail2ban returns UnknownJailException.
+
+        fail2ban serialises the exception without a space (``UnknownJailException``
+        rather than ``Unknown JailException``), so _is_not_found_error must match
+        the concatenated form ``"unknownjail``".
+        """
+        response = (1, Exception("UnknownJailException('blocklist-import')"))
+        with (
+            _patch_client({"set|missing-jail|banip|1.2.3.4": response}),
+            pytest.raises(JailNotFoundError, match="missing-jail"),
+        ):
+            await jail_service.ban_ip(_SOCKET, "missing-jail", "1.2.3.4")
+
     async def test_ban_ipv6_success(self) -> None:
         """ban_ip accepts an IPv6 address."""
         with _patch_client({"set|sshd|banip|::1": (0, 1)}):