diff --git a/CHANGELOG.md b/CHANGELOG.md
index 35e927f9..2c02b7df 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -233,6 +233,14 @@ adheres to [Semantic Versioning](https://semver.org/).
 - `apply_privacy_policy` and `classify_columns` raise `ValueError` for
   duplicate column labels or labels that collide as strings, such as `1` and
   `"1"` (#232, part 5).
+- A `MaskingRule` column name matches every column with the same name or the
+  same snake_case form, so `"First Name"` masks `first_name` and `"email"`
+  masks both `email` and `Email`. Listed names that match no column are
+  recorded under a new `MaskReport.unmatched_columns` key; they raise
+  `ValueError` only with `MaskingRule(strict=True)` or
+  `mask_dataframe(..., strict=True)`. `freshdata clean --mask COLUMN:STRATEGY`
+  rules are strict, so it exits 1 with a one-line error when `COLUMN` matches
+  no column, instead of masking nothing and exiting 0 (#251).
 
 ### Fixed
 - Trust-gate integrations now validate `on_low_score` policies at configuration
diff --git a/src/freshdata/enterprise/cleaner.py b/src/freshdata/enterprise/cleaner.py
index f8dd9762..030dedae 100644
--- a/src/freshdata/enterprise/cleaner.py
+++ b/src/freshdata/enterprise/cleaner.py
@@ -32,6 +32,7 @@
 
 from .._util import _is_stringlike_dtype
 from ..adapters.polars import _polars_module, is_polars_frame, to_pandas
+from ..steps.columns import snake_case
 from .config import ClusterConfig, MaskingRule, SemanticValidatorConfig
 
 
@@ -342,11 +343,53 @@ def _scrub_patterns(rule: MaskingRule) -> list[str]:
     return [PII_PATTERNS[name] for name in rule.scrub_patterns] + list(rule.regexes)
 
 
-def _resolve_columns(rule: MaskingRule, columns: Sequence) -> list:
-    selected = [c for c in columns if c in set(rule.columns)]
+def _resolve_columns(
+    rule: MaskingRule,
+    columns: Sequence,
+    report: MaskReport | None = None,
+    strict: bool | None = None,
+) -> list:
+    """Return the columns *rule* selects from *columns*.
+
+    A listed column matches a frame column with the same name or the same
+    snake_case form. Every matching column is selected, so a rule never masks
+    one of two columns that normalise to the same name and skips the other.
+    Listed columns that match nothing are recorded on *report*; they raise
+    ``ValueError`` only when *strict* (or ``rule.strict`` when *strict* is None).
+    """
+    col_list = list(columns)
+    col_by_snake: dict[str, list] = {}
+    for c in col_list:
+        col_by_snake.setdefault(snake_case(str(c)), []).append(c)
+
+    selected: list = []
+    unmatched: list = []
+
+    for name in rule.columns:
+        matches = [c for c in col_list if c == name]
+        key = snake_case(str(name))
+        if key:
+            matches += [c for c in col_by_snake.get(key, []) if c not in matches]
+        if not matches:
+            unmatched.append(name)
+        selected += [c for c in matches if c not in selected]
+
     if rule.pattern:
         regex = re.compile(rule.pattern)
-        selected += [c for c in columns if c not in selected and regex.search(str(c))]
+        selected += [c for c in col_list if c not in selected and regex.search(str(c))]
+
+    if report is not None:
+        for u in unmatched:
+            if str(u) not in report.unmatched_columns:
+                report.unmatched_columns.append(str(u))
+
+    is_strict = rule.strict if strict is None else strict
+    if is_strict and unmatched:
+        missing = ", ".join(repr(c) for c in unmatched)
+        raise ValueError(
+            f"Masking rule {rule.name!r} specifies column(s) not found in dataframe: {missing}"
+        )
+
     return selected
 
 
@@ -363,6 +406,8 @@ class MaskReport:
     #: Auditable provenance: one entry per masked column recording *which* rule
     #: masked it, with what strategy, under which policy id, and why.
     policy_provenance: list[dict[str, Any]] = field(default_factory=list)
+    #: Columns explicitly specified by rules that did not match any dataframe column.
+    unmatched_columns: list[str] = field(default_factory=list)
 
     def _record(self, column: str, rule: MaskingRule, n_cells: int) -> None:
         self.columns[column] = rule.strategy
@@ -393,6 +438,7 @@ def to_dict(self) -> dict[str, Any]:
             "rules_applied": list(self.rules_applied),
             "retention": dict(self.retention),
             "policy_provenance": list(self.policy_provenance),
+            "unmatched_columns": list(self.unmatched_columns),
         }
 
     def __repr__(self) -> str:
@@ -464,7 +510,11 @@ def _apply_mask_pandas(
     return out, changed
 
 
-def mask_dataframe(df: Any, rules: Sequence[MaskingRule]) -> tuple[Any, MaskReport]:
+def mask_dataframe(
+    df: Any,
+    rules: Sequence[MaskingRule],
+    strict: bool | None = None,
+) -> tuple[Any, MaskReport]:
     """Apply PII masking *rules* to *df*; returns ``(df_same_type, MaskReport)``.
 
     Rules run in order; ``drop`` removes the column so later rules see the new schema.
@@ -478,7 +528,8 @@ def mask_dataframe(df: Any, rules: Sequence[MaskingRule]) -> tuple[Any, MaskRepo
                 f"strategy {rule.strategy!r} is a privacy strategy; apply it with "
                 "freshdata.enterprise.privacy.anonymize (or fd.anonymize), not mask_dataframe"
             )
-        for column in _resolve_columns(rule, _all_columns(out)):
+        resolved = _resolve_columns(rule, _all_columns(out), report=report, strict=strict)
+        for column in resolved:
             if column not in _all_columns(out):
                 continue
             if is_polars_frame(out):
diff --git a/src/freshdata/enterprise/cli.py b/src/freshdata/enterprise/cli.py
index fcf9cbd2..c7a4ae03 100644
--- a/src/freshdata/enterprise/cli.py
+++ b/src/freshdata/enterprise/cli.py
@@ -258,8 +258,14 @@ def cmd_clean(args: argparse.Namespace) -> int:
     extra_masks = []
     for spec in args.mask or []:
         column, _, strategy = spec.partition(":")
+        # A column named on the command line must exist, so these rules are strict.
         extra_masks.append(
-            MaskingRule(name=f"cli_{column}", columns=(column,), strategy=strategy or "hash")
+            MaskingRule(
+                name=f"cli_{column}",
+                columns=(column,),
+                strategy=strategy or "hash",
+                strict=True,
+            )
         )
     masking = tuple(ec.masking) + tuple(extra_masks)
 
diff --git a/src/freshdata/enterprise/config.py b/src/freshdata/enterprise/config.py
index 4d9762c8..84b111ba 100644
--- a/src/freshdata/enterprise/config.py
+++ b/src/freshdata/enterprise/config.py
@@ -42,9 +42,12 @@
 class MaskingRule:
     """One PII masking rule applied to a set of columns.
 
-    Columns are selected by exact ``columns`` names (post-clean snake_case) and
-    by ``pattern`` (a regex matched against column *names*). At least one
-    selector must be given.
+    Columns are selected by ``columns`` names and by ``pattern`` (a regex
+    matched against column *names*). At least one selector must be given. A
+    listed name matches a column with the same name or the same snake_case
+    form (``"First Name"`` matches ``first_name``), and every column that
+    matches is masked. Listed names that match nothing are reported, and raise
+    only when ``strict=True``.
 
     Strategies
     ----------
@@ -107,6 +110,11 @@ class MaskingRule:
     policy_id: str | None = None
     #: Human-readable justification recorded alongside ``policy_id`` (the *why*).
     policy_reason: str | None = None
+    #: When True, raise ``ValueError`` if a listed column matches no column of the
+    #: frame being masked. The default (False) records it in
+    #: ``MaskReport.unmatched_columns`` instead, so one rule set can be reused
+    #: across frames that don't all have every column.
+    strict: bool = False
 
     def __post_init__(self) -> None:
         # ``token`` is an accepted alias for the reversible ``tokenize`` strategy.
@@ -138,6 +146,7 @@ def __post_init__(self) -> None:
         object.__setattr__(self, "entity_types", tuple(self.entity_types))
         object.__setattr__(self, "hipaa_tags", tuple(self.hipaa_tags))
         object.__setattr__(self, "gdpr_tags", tuple(self.gdpr_tags))
+        object.__setattr__(self, "strict", bool(self.strict))
         # Secure default: an empty salt on a hash rule would make low-entropy
         # PII trivially reversible, so generate a random per-rule salt instead.
         if self.strategy == "hash" and not self.salt:
diff --git a/src/freshdata/enterprise/privacy.py b/src/freshdata/enterprise/privacy.py
index 3ebcd523..84dc740f 100644
--- a/src/freshdata/enterprise/privacy.py
+++ b/src/freshdata/enterprise/privacy.py
@@ -1133,7 +1133,9 @@ def anonymize(
                 f"duplicated: {duplicated}"
             )
         for rule in rules:
-            targeted = _resolve_columns(rule, duplicated)
+            # Only the duplicated labels are searched here, so a listed column
+            # that is absent from them is expected; never raise for it.
+            targeted = _resolve_columns(rule, duplicated, strict=False)
             if targeted:
                 raise ValueError(
                     f"anonymize requires unique column labels; rule {rule.name!r} "
diff --git a/tests/test_enterprise_cleaner.py b/tests/test_enterprise_cleaner.py
index 8f955b49..50f5dfa2 100644
--- a/tests/test_enterprise_cleaner.py
+++ b/tests/test_enterprise_cleaner.py
@@ -3,6 +3,7 @@
 import pandas as pd
 import pytest
 
+import freshdata as fd
 from freshdata.adapters.polars import is_polars_frame
 from freshdata.enterprise import (
     PII_PATTERNS,
@@ -224,6 +225,101 @@ def test_pii_patterns_present():
     assert {"email", "phone", "ssn", "credit_card", "ip", "iban"} <= set(PII_PATTERNS)
 
 
+def test_masking_snake_case_column_matching():
+    df = pd.DataFrame({"email": ["a@x.com"], "first_name": ["Alice"]})
+    out, report = mask_dataframe(
+        df,
+        [
+            MaskingRule(name="m1", columns=("Email",), strategy="hash"),
+            MaskingRule(
+                name="m2", columns=("First Name",), strategy="redact", placeholder="[REDACTED]"
+            ),
+        ],
+    )
+    assert out["email"].iloc[0] != "a@x.com"
+    assert out["first_name"].iloc[0] == "[REDACTED]"
+    assert report.columns["email"] == "hash"
+    assert report.columns["first_name"] == "redact"
+    assert report.unmatched_columns == []
+
+
+def test_masking_missing_column_strict_raises():
+    df = pd.DataFrame({"email": ["a@x.com"]})
+    rule = MaskingRule(name="m", columns=("ghost",), strategy="hash", strict=True)
+    with pytest.raises(ValueError, match=r"not found in dataframe.*'ghost'"):
+        mask_dataframe(df, [rule])
+
+
+def test_masking_missing_column_is_recorded_by_default():
+    df = pd.DataFrame({"email": ["a@x.com"]})
+    rule = MaskingRule(name="m", columns=("ghost",), strategy="hash")
+    assert rule.strict is False
+    out, report = mask_dataframe(df, [rule])
+    assert out["email"].iloc[0] == "a@x.com"
+    assert report.unmatched_columns == ["ghost"]
+    assert report.to_dict()["unmatched_columns"] == ["ghost"]
+
+
+def test_mask_dataframe_strict_argument_overrides_rule():
+    df = pd.DataFrame({"email": ["a@x.com"]})
+    with pytest.raises(ValueError, match="'ghost'"):
+        mask_dataframe(df, [MaskingRule(name="m", columns=("ghost",))], strict=True)
+    strict_rule = MaskingRule(name="m", columns=("ghost",), strict=True)
+    _, report = mask_dataframe(df, [strict_rule], strict=False)
+    assert report.unmatched_columns == ["ghost"]
+
+
+def test_masking_masks_every_column_with_the_same_snake_case_name():
+    df = pd.DataFrame({"E-mail": ["a@x.com"], "e_mail": ["b@y.com"]})
+    rule = MaskingRule(name="r", columns=("E Mail",), strategy="redact", placeholder="X")
+    out, report = mask_dataframe(df, [rule])
+    assert out["E-mail"].iloc[0] == "X"
+    assert out["e_mail"].iloc[0] == "X"
+    assert set(report.columns) == {"E-mail", "e_mail"}
+
+
+def test_masking_exact_name_also_masks_snake_case_twin():
+    df = pd.DataFrame({"email": ["a@x.com"], "Email": ["b@y.com"]})
+    rule = MaskingRule(name="r", columns=("email",), strategy="redact", placeholder="X")
+    out, _ = mask_dataframe(df, [rule])
+    assert out["email"].iloc[0] == "X"
+    assert out["Email"].iloc[0] == "X"
+
+
+def test_anonymize_shared_rule_with_absent_column_does_not_raise():
+    df = pd.DataFrame({"email": ["a@x.com"]})
+    rule = MaskingRule(name="r", columns=("email", "ssn"), strategy="redact", placeholder="X")
+    out, _ = fd.anonymize(df, rules=(rule,))
+    assert out["email"].iloc[0] == "X"
+
+
+def test_anonymize_strict_rule_with_duplicated_labels_elsewhere():
+    df = pd.DataFrame([["a@x.com", 1, 2]], columns=["email", "dup", "dup"])
+    rule = MaskingRule(
+        name="r", columns=("email",), strategy="redact", placeholder="X", strict=True
+    )
+    out, _ = fd.anonymize(df, rules=(rule,))
+    assert out["email"].iloc[0] == "X"
+
+
+def test_masking_polars_snake_case_matching_masks_every_match():
+    pl = pytest.importorskip("polars")
+    frame = pl.DataFrame(
+        {"first_name": ["Alice"], "email": ["a@x.com"], "Email": ["b@y.com"]}
+    )
+    rules = [
+        MaskingRule(name="n", columns=("First Name",), strategy="redact", placeholder="X"),
+        MaskingRule(name="e", columns=("email",), strategy="redact", placeholder="X"),
+    ]
+    out, report = mask_dataframe(frame, rules)
+    assert is_polars_frame(out)
+    assert out["first_name"][0] == "X"
+    assert out["email"][0] == "X"
+    assert out["Email"][0] == "X"
+    assert report.columns == {"first_name": "redact", "email": "redact", "Email": "redact"}
+    assert report.unmatched_columns == []
+
+
 # =====================================================================
 # Semantic validation
 # =====================================================================
diff --git a/tests/test_enterprise_cli.py b/tests/test_enterprise_cli.py
index c8807cff..d3d36be8 100644
--- a/tests/test_enterprise_cli.py
+++ b/tests/test_enterprise_cli.py
@@ -192,3 +192,25 @@ def test_unreadable_config_prints_one_line_error_not_traceback(tmp_path, capsys)
     code = cli.main(["clean", str(src), "-o", str(tmp_path / "o.csv"), "--config", str(cfg)])
     assert code == 1
     assert "Traceback" not in capsys.readouterr().err
+
+
+def test_mask_missing_column_prints_one_line_error(tmp_path, capsys):
+    src = tmp_path / "in.csv"
+    pd.DataFrame({"email": ["a@x.com"]}).to_csv(src, index=False)
+    out = tmp_path / "o.csv"
+    code = cli.main(["clean", str(src), "-o", str(out), "--mask", "non_existent:hash"])
+    err = capsys.readouterr().err
+    assert code == 1
+    assert "not found in dataframe" in err
+    assert "non_existent" in err
+    assert "Traceback" not in err
+
+
+def test_mask_case_normalization_in_cli(tmp_path):
+    src = tmp_path / "in.csv"
+    pd.DataFrame({"email": ["a@x.com"]}).to_csv(src, index=False)
+    out = tmp_path / "o.csv"
+    code = cli.main(["clean", str(src), "-o", str(out), "--mask", "Email:hash", "--quiet"])
+    assert code == 0
+    assert pd.read_csv(out)["email"].iloc[0] != "a@x.com"
+
diff --git a/tests/test_enterprise_interface.py b/tests/test_enterprise_interface.py
index 7e9251e9..63258a6e 100644
--- a/tests/test_enterprise_interface.py
+++ b/tests/test_enterprise_interface.py
@@ -157,3 +157,31 @@ def test_cli_clean_fails_closed_on_anonymization(raw, tmp_path, monkeypatch, cap
     assert code != 0
     assert "anonymization is not supported" in capsys.readouterr().err
     assert not out.exists()
+
+
+# -- Masking rules naming a missing column (#251) --------------------------
+
+
+def _missing_column_config(*, strict):
+    rule = MaskingRule(
+        name="pii",
+        columns=("email", "ghost"),
+        strategy="redact",
+        placeholder="X",
+        strict=strict,
+    )
+    return EnterpriseConfig(masking=(rule,))
+
+
+def test_clean_enterprise_strict_masking_rule_raises_for_missing_column(raw):
+    with pytest.raises(ValueError, match=r"'pii'.*not found in dataframe.*'ghost'"):
+        clean_enterprise(raw, enterprise=_missing_column_config(strict=True), verbose=False)
+
+
+def test_clean_enterprise_non_strict_masking_rule_reports_missing_column(raw):
+    result = clean_enterprise(raw, enterprise=_missing_column_config(strict=False), verbose=False)
+    assert (result.data["email"] == "X").all()
+    assert result.privacy_report is None
+    assert result.mask_report is not None
+    assert result.mask_report.unmatched_columns == ["ghost"]
+    assert result.mask_report.to_dict()["unmatched_columns"] == ["ghost"]