diff --git a/tests/integration/_support/mail.py b/tests/integration/_support/mail.py
index ef4f8f9a0ca..56c74193152 100644
--- a/tests/integration/_support/mail.py
+++ b/tests/integration/_support/mail.py
@@ -2,6 +2,7 @@ from __future__ import annotations
 
 import socketserver
 import threading
+from collections import deque
 from collections.abc import Generator
 from contextlib import contextmanager
 from dataclasses import dataclass
@@ -33,13 +34,10 @@ class Delivery:
 class Mailbox:
     host: str
     port: int
-    received: SimpleQueue[Delivery]
+    received: deque[Delivery]
 
-    def pending(self) -> int:
-        return self.received.qsize()
-
-    def drain(self) -> tuple[Delivery, ...]:
-        return tuple(self.received.get_nowait() for _ in range(self.received.qsize()))
+    def with_subject(self, fragment: str) -> tuple[Delivery, ...]:
+        return tuple(delivery for delivery in tuple(self.received) if fragment in delivery.subject)
 
 
 def _address(argument: str) -> str:
@@ -49,7 +47,7 @@ def _address(argument: str) -> str:
 @contextmanager
 def smtp_sink() -> Generator[Mailbox, None, None]:
     """Owned plaintext SMTP peer; deliveries traverse the proxy's real smtplib client."""
-    received: Final[SimpleQueue[Delivery]] = SimpleQueue()
+    received: Final[deque[Delivery]] = deque()  # mutable-ok: sink thread appends each delivery
     errors: Final[SimpleQueue[Exception]] = SimpleQueue()
 
     class Handler(socketserver.StreamRequestHandler):
@@ -92,7 +90,7 @@ def smtp_sink() -> Generator[Mailbox, None, None]:
                         if not chunk or chunk == b".\r\n":
                             break
                         body.extend(chunk[1:] if chunk.startswith(b"..") else chunk)
-                    received.put(Delivery(sender, recipients, message_from_bytes(bytes(body))))
+                    received.append(Delivery(sender, recipients, message_from_bytes(bytes(body))))
                     sender, recipients = "", ()
                     self._reply("250 OK queued")
                 elif verb == "RSET":
diff --git a/tests/integration/spend/test_team_member_budget_alerts.py b/tests/integration/spend/test_team_member_budget_alerts.py
index 49c3837aefa..5aedad3e360 100644
--- a/tests/integration/spend/test_team_member_budget_alerts.py
+++ b/tests/integration/spend/test_team_member_budget_alerts.py
@@ -56,7 +56,7 @@ def test_team_member_budget_thresholds_email_member_and_configured_recipients(ga
             eventually(
                 lambda: _membership_spend(user_id, team_id), lambda spend: spend == pytest.approx(CALL_COST), seconds=70
             )
-            assert mailbox.drain() == (), "no threshold is reached before the first call is recorded"
+            assert tuple(mailbox.received) == (), "no threshold is reached before the first call is recorded"
 
             second: Final = candidate.request(
                 "POST",
@@ -65,10 +65,8 @@ def test_team_member_budget_thresholds_email_member_and_configured_recipients(ga
                 key=key,
             )
             assert second.status_code == 200, second.text
-            eventually(mailbox.pending, lambda count: count >= 1, seconds=30)
-            halfway: Final = mailbox.drain()
+            halfway: Final = eventually(lambda: mailbox.with_subject("50%"), lambda found: len(found) >= 1, seconds=30)
             assert [delivery.recipients for delivery in halfway] == [(member_email,)], halfway
-            assert "50%" in halfway[0].subject, halfway[0].subject
             assert f"${MEMBER_BUDGET}" in halfway[0].html, halfway[0].html
             eventually(
                 lambda: _membership_spend(user_id, team_id),
@@ -83,9 +81,7 @@ def test_team_member_budget_thresholds_email_member_and_configured_recipients(ga
                 key=key,
             )
             assert third.status_code == 422 and third.json()["error"]["type"] == "budget_exceeded", third.text
-            eventually(mailbox.pending, lambda count: count >= 2, seconds=30)
-            hundred: Final = mailbox.drain()
-            assert all("100%" in delivery.subject for delivery in hundred), hundred
+            hundred: Final = eventually(lambda: mailbox.with_subject("100%"), lambda found: len(found) >= 2, seconds=30)
             assert {recipient for delivery in hundred for recipient in delivery.recipients} == {
                 member_email,
                 finance_email,