diff --git a/backend/app/platform/config.py b/backend/app/platform/config.py
index 70eae74..70251e7 100644
--- a/backend/app/platform/config.py
+++ b/backend/app/platform/config.py
@@ -36,5 +36,35 @@ def cookie_secure() -> bool:
 
 
 def mailer_kind() -> str:
-    """Which mailer adapter to build: 'log' (dev/tests) or 'smtp' (deployed; SLICE-4)."""
+    """Which mailer adapter to build: 'log' (dev/tests) or 'smtp' (deployed)."""
     return os.environ.get("ECOMM_MAILER") or "log"
+
+
+# SMTP relay coordinates (deployed envs only; INV-8 — host/port/user/from are non-secret
+# overlay values, the password is a Secret Manager reference resolved by the deploy).
+
+
+def smtp_host() -> str:
+    return os.environ.get("ECOMM_SMTP_HOST", "")
+
+
+def smtp_port() -> int:
+    return int(os.environ.get("ECOMM_SMTP_PORT") or "587")
+
+
+def smtp_user() -> str:
+    return os.environ.get("ECOMM_SMTP_USER", "")
+
+
+def smtp_password() -> str:
+    return os.environ.get("ECOMM_SMTP_PASSWORD", "")
+
+
+def smtp_from() -> str:
+    """The From header; defaults to the relay user."""
+    return os.environ.get("ECOMM_SMTP_FROM") or smtp_user()
+
+
+def smtp_starttls() -> bool:
+    """STARTTLS on the relay connection (default on; disable only for odd relays)."""
+    return os.environ.get("ECOMM_SMTP_STARTTLS", "1").strip().lower() not in {"0", "false", "no", "off"}
diff --git a/backend/app/platform/mailer.py b/backend/app/platform/mailer.py
index f38eb66..92dae36 100644
--- a/backend/app/platform/mailer.py
+++ b/backend/app/platform/mailer.py
@@ -2,20 +2,30 @@
 
 A one-method port `send(to, subject, body)` with two adapters: `LogMailer` (dev/tests —
 the message lands in the app log and an in-memory outbox tests read back, §6.8) and the
-deployed `SmtpMailer` (SLICE-4). The adapter is chosen by configuration at startup
-(INV-8), so no deployment shape lives in the domain. LogMailer logs the full body on
-purpose: that is PUC-10's "local dev channel" — the one-time code reaches the developer in
-the terminal. The deployed mailer (SLICE-4) will NOT log the body (INV-3 / §6.6 log hygiene).
+deployed `SmtpMailer` (relay coordinates from configuration, INV-8). The adapter is chosen
+by configuration at startup, so no deployment shape lives in the domain. LogMailer logs the
+full body on purpose: that is PUC-10's "local dev channel" — the one-time code reaches the
+developer in the terminal. SmtpMailer never logs the body or the recipient (INV-3 / §6.6
+log hygiene) and raises MailerError on delivery failure (INV-9 → the §6.4 502).
 """
 from __future__ import annotations
 
+import hashlib
 import logging
+import smtplib
 from dataclasses import dataclass
+from email.message import EmailMessage
 from typing import Protocol
 
+from app.platform import config
+
 logger = logging.getLogger("ecomm.mailer")
 
 
+class MailerError(Exception):
+    """Delivery failed — the relay refused or was unreachable (INV-9)."""
+
+
 @dataclass(frozen=True)
 class SentMessage:
     to: str
@@ -41,10 +51,48 @@ class LogMailer:
         logger.info("LogMailer -> %s | %s\n%s", to, subject, body)
 
 
+class SmtpMailer:
+    """Deployed adapter: real mail over an SMTP relay (STARTTLS + login by default).
+
+    Logs only a hashed recipient — never the address, subject, or body (§6.6).
+    """
+
+    def __init__(self, host: str, port: int, user: str, password: str, sender: str, starttls: bool) -> None:
+        self._host, self._port = host, port
+        self._user, self._password = user, password
+        self._sender, self._starttls = sender, starttls
+
+    def send(self, to: str, subject: str, body: str) -> None:
+        msg = EmailMessage()
+        msg["To"] = to
+        msg["From"] = self._sender
+        msg["Subject"] = subject
+        msg.set_content(body)
+        to_hash = hashlib.sha256(to.encode("utf-8")).hexdigest()[:8]
+        try:
+            with smtplib.SMTP(self._host, self._port, timeout=10) as smtp:
+                if self._starttls:
+                    smtp.starttls()
+                if self._user:
+                    smtp.login(self._user, self._password)
+                smtp.send_message(msg)
+        except Exception as exc:
+            logger.warning("smtp send FAILED to=%s: %s", to_hash, type(exc).__name__)
+            raise MailerError(str(exc)) from exc
+        logger.info("smtp sent to=%s", to_hash)
+
+
 def build_mailer(kind: str) -> Mailer:
     """Select the mailer adapter by configured kind (config.mailer_kind(), INV-8)."""
     if kind == "log":
         return LogMailer()
     if kind == "smtp":
-        raise NotImplementedError("SmtpMailer arrives in SLICE-4 (deployed environments)")
+        return SmtpMailer(
+            host=config.smtp_host(),
+            port=config.smtp_port(),
+            user=config.smtp_user(),
+            password=config.smtp_password(),
+            sender=config.smtp_from(),
+            starttls=config.smtp_starttls(),
+        )
     raise ValueError(f"unknown mailer kind: {kind!r}")
diff --git a/backend/tests/test_mailer.py b/backend/tests/test_mailer.py
index 961bd4e..7eee087 100644
--- a/backend/tests/test_mailer.py
+++ b/backend/tests/test_mailer.py
@@ -17,10 +17,95 @@ def test_build_mailer_log_kind():
     assert isinstance(m, mailer.LogMailer)
 
 
-def test_build_mailer_smtp_not_yet_available():
-    # SmtpMailer lands in SLICE-4; until then asking for it is an explicit error, not a
-    # silent fallback to LogMailer (which would send no real mail in a deployed env).
+class _FakeSMTP:
+    """Stand-in for smtplib.SMTP capturing the call sequence (no network)."""
+
+    instances: list["_FakeSMTP"] = []
+    fail_on_send = False
+
+    def __init__(self, host, port, timeout=None):
+        self.host, self.port, self.timeout = host, port, timeout
+        self.calls: list[str] = []
+        self.message = None
+        _FakeSMTP.instances.append(self)
+
+    def __enter__(self):
+        return self
+
+    def __exit__(self, *exc):
+        return False
+
+    def starttls(self):
+        self.calls.append("starttls")
+
+    def login(self, user, password):
+        self.calls.append(f"login:{user}")
+
+    def send_message(self, msg):
+        if _FakeSMTP.fail_on_send:
+            raise RuntimeError("relay refused")
+        self.calls.append("send_message")
+        self.message = msg
+
+
+def _smtp_env(monkeypatch):
+    monkeypatch.setattr(mailer.smtplib, "SMTP", _FakeSMTP)
+    _FakeSMTP.instances.clear()
+    _FakeSMTP.fail_on_send = False
+    monkeypatch.setenv("ECOMM_SMTP_HOST", "smtp.example.com")
+    monkeypatch.setenv("ECOMM_SMTP_PORT", "587")
+    monkeypatch.setenv("ECOMM_SMTP_USER", "sender@example.com")
+    monkeypatch.setenv("ECOMM_SMTP_PASSWORD", "not-a-real-password")
+    monkeypatch.setenv("ECOMM_SMTP_FROM", "ecomm <sender@example.com>")
+
+
+def test_build_mailer_smtp_builds_from_config(monkeypatch):
+    _smtp_env(monkeypatch)
+    m = mailer.build_mailer("smtp")
+    assert isinstance(m, mailer.SmtpMailer)
+
+
+def test_smtpmailer_sends_via_starttls_login(monkeypatch):
+    _smtp_env(monkeypatch)
+    m = mailer.build_mailer("smtp")
+    m.send("merchant@example.com", "Your ecomm code: 123456", "Code: 123456")
+    smtp = _FakeSMTP.instances[-1]
+    assert (smtp.host, smtp.port) == ("smtp.example.com", 587)
+    assert smtp.calls == ["starttls", "login:sender@example.com", "send_message"]
+    assert smtp.message["To"] == "merchant@example.com"
+    assert smtp.message["Subject"] == "Your ecomm code: 123456"
+    assert smtp.message["From"] == "ecomm <sender@example.com>"
+    assert "123456" in smtp.message.get_content()
+
+
+def test_smtpmailer_failure_raises_mailer_error(monkeypatch):
+    _smtp_env(monkeypatch)
+    _FakeSMTP.fail_on_send = True
     import pytest
 
-    with pytest.raises(NotImplementedError):
-        mailer.build_mailer("smtp")
+    m = mailer.build_mailer("smtp")
+    with pytest.raises(mailer.MailerError):
+        m.send("merchant@example.com", "subject", "body")
+
+
+def test_smtpmailer_never_logs_the_body(monkeypatch):
+    # §6.6 log hygiene: codes never appear in deployed logs. LogMailer logging the body
+    # is dev-only by configuration; the deployed adapter must not. The handler attaches
+    # directly to the ecomm.mailer logger (it doesn't propagate to root).
+    import logging
+
+    _smtp_env(monkeypatch)
+    records: list[str] = []
+    handler = logging.Handler()
+    handler.emit = lambda r: records.append(r.getMessage())  # type: ignore[method-assign]
+    lg = logging.getLogger("ecomm.mailer")
+    lg.addHandler(handler)
+    lg.setLevel(logging.DEBUG)
+    try:
+        mailer.build_mailer("smtp").send(
+            "merchant@example.com", "Your ecomm code: 123456", "Code: 123456"
+        )
+    finally:
+        lg.removeHandler(handler)
+    assert records, "the deployed adapter should log the send event (observability §6.6)"
+    assert all("123456" not in r and "merchant@example.com" not in r for r in records)