diff --git a/scripts/build_healthcare_campaigns_cron.py b/scripts/build_healthcare_campaigns_cron.py
index faf3049..dffb7ac 100644
--- a/scripts/build_healthcare_campaigns_cron.py
+++ b/scripts/build_healthcare_campaigns_cron.py
@@ -131,6 +131,54 @@ def daily_slice(day: int) -> int:
     return 1000
 
 
+# ── Per-MX-operator throttle ─────────────────────────────────────────────────
+# Sender reputation is tracked by the RECEIVING mail operator (Microsoft 365,
+# Google Workspace, Proofpoint, ...), not by recipient domain. So we cap how many
+# new providers we queue per operator per day, and let volume spread across the
+# long tail of operators freely. This lets total daily volume be much higher than
+# a flat cap without hammering any single receiving system. Caps ramp with the
+# warmup day. "default" applies to any operator not explicitly listed (mostly the
+# long tail of small/independent mail hosts -- a generous cap is safe there
+# because each sees only a handful).
+def mx_daily_caps(day: int) -> dict:
+    # (microsoft, google, proofpoint, default-per-operator)
+    if day <= 1:   big, default = 25, 15
+    elif day <= 4: big, default = 60, 40
+    elif day <= 9: big, default = 120, 80
+    else:          big, default = 250, 150
+    return {
+        "microsoft": big,
+        "google": big,
+        "proofpoint": big,
+        "cisco": big,
+        "mimecast": big,
+        "barracuda": big,
+        "__default__": default,
+    }
+
+
+def mx_throttled(candidates: list[dict], total_n: int, caps: dict) -> list[dict]:
+    """Pick up to total_n candidates, capping per mx_provider so no single
+    receiving operator gets more than its daily share. Preserves input order
+    within each operator. Falls back to ungrouped slicing if rows have no
+    mx_provider."""
+    if not candidates or "mx_provider" not in candidates[0]:
+        return candidates[:total_n]
+    per_op: dict = {}
+    chosen: list[dict] = []
+    default_cap = caps.get("__default__", 50)
+    for r in candidates:
+        if len(chosen) >= total_n:
+            break
+        op = (r.get("mx_provider") or "").strip() or "__default__"
+        cap = caps.get(op, default_cap)
+        if per_op.get(op, 0) >= cap:
+            continue
+        per_op[op] = per_op.get(op, 0) + 1
+        chosen.append(r)
+    return chosen
+
+
 def lm(path: str, data=None, method=None):
     tok = _token()
     headers = {"Content-Type": "application/json",
@@ -334,7 +382,9 @@ def warm_segment(seg_key: str, rows: list[dict], slice_n: int,
                   and r["email"].strip().lower() not in suppressed
                   and not _is_google_hosted(r)
                   and row_matches(seg_key, r)]
-    todo = candidates[:slice_n]
+    # Spread the slice across MX operators so no single receiving system (e.g.
+    # Microsoft 365) gets the whole batch. Caps ramp with the warmup day.
+    todo = mx_throttled(candidates, slice_n, mx_daily_caps(warmup_day()))
     print(f"[hc-cron]   {seg_key}: candidates={len(candidates)} "
           f"already={len(imported)} to_import={len(todo)}")