diff --git a/files/srs/srs-alias-decision.py b/files/srs/srs-alias-decision.py
new file mode 100755
index 0000000..190d624
--- /dev/null
+++ b/files/srs/srs-alias-decision.py
@@ -0,0 +1,258 @@
+#!/usr/bin/python3
+
+import subprocess
+from collections import deque
+import sys
+
+ALIAS_MAPS = [
+    "pgsql:/etc/postfix/pgsql_virtual_alias_maps.cf",
+    "pgsql:/etc/postfix/pgsql_virtual_alias_domain_maps.cf",
+    "pgsql:/etc/postfix/pgsql_virtual_alias_address_rewriting.cf",
+    "pgsql:/etc/postfix/pgsql_virtual_alias_domain_catchall_maps.cf",
+    "btree:/etc/postfix/virtual_alias_maps",
+]
+
+MAILBOX_MAPS = [
+    "pgsql:/etc/postfix/pgsql_virtual_mailbox_maps.cf",
+    "pgsql:/etc/postfix/pgsql_virtual_alias_domain_mailbox_maps.cf",
+]
+
+DOMAIN_MAPS = [
+    "pgsql:/etc/postfix/pgsql_virtual_domains_maps.cf",
+    "pgsql:/etc/postfix/pgsql_relay-domain-maps.cf",
+    "btree:/etc/postfix/virtual_alias_domains",
+]
+
+TRANSPORT_MAPS = [
+    "pgsql:/etc/postfix/pgsql_transport_maps.cf",
+    "btree:/etc/postfix/transport",
+    "btree:/etc/postfix/relay_domains",
+]
+
+
+def lookup(key, table):
+    result = subprocess.run(
+        ["/usr/sbin/postmap", "-q", key, table],
+        capture_output=True,
+        text=True,
+        timeout=10,
+    )
+
+    if result.returncode == 0:
+        return result.stdout.strip() or None
+
+    if result.returncode == 1 and not result.stderr.strip():
+        return None
+
+    raise RuntimeError(
+        f"{table}: {result.stderr.strip() or 'Lookup fehlgeschlagen'}"
+    )
+
+
+def lookup_any(key, tables):
+    for table in tables:
+        value = lookup(key, table)
+        if value is not None:
+            return value
+    return None
+
+
+def postconf(name):
+    result = subprocess.run(
+        ["/usr/sbin/postconf", "-c", "/etc/postfix", "-x", "-h", name],
+        capture_output=True,
+        text=True,
+        timeout=10,
+    )
+
+    if result.returncode:
+        raise RuntimeError(result.stderr.strip())
+
+    return result.stdout.strip()
+
+
+def transport_keys(address):
+    localpart, domain = address.rsplit("@", 1)
+
+    keys = [address, f"{localpart}@"]
+
+    parts = domain.split(".")
+
+    for i in range(len(parts)):
+        suffix = ".".join(parts[i:])
+        keys.extend([suffix, "." + suffix])
+
+    return list(dict.fromkeys(keys))
+
+
+def classify_destination(address):
+    domain = address.rsplit("@", 1)[1]
+
+    if lookup_any(address, MAILBOX_MAPS):
+        return "LOCAL"
+
+    if domain in postconf("mydestination").lower().split():
+        return "LOCAL"
+
+    if lookup_any(domain, DOMAIN_MAPS):
+        return "UNKNOWN"
+
+    for key in transport_keys(address):
+        transport = lookup_any(key, TRANSPORT_MAPS)
+
+        if transport:
+            transport_type = transport.split(":", 1)[0].lower()
+
+            if transport_type in ("smtp", "srs-forward"):
+                return "EXTERNAL"
+
+            return "UNKNOWN"
+
+    default_transport = postconf("default_transport").strip().lower()
+    if (
+        default_transport in ("smtp", "srs-forward")
+        or default_transport.startswith(("smtp:", "srs-forward:"))
+    ):
+        return "EXTERNAL"
+
+    return "UNKNOWN"
+
+
+MAX_ALIAS_DEPTH = 10
+MAX_ALIAS_NODES = 200
+
+
+def alias_reaches(original, recipient):
+    """Find alias paths with BFS and detect cycles in the explored graph."""
+    pending = deque([(original, 0)])
+    visited = {original}
+    edges = {}
+    nodes = 0
+    depth_exceeded = False
+
+    while pending:
+        current, depth = pending.popleft()
+        nodes += 1
+
+        if nodes > MAX_ALIAS_NODES:
+            return None, "Zu viele Alias-Knoten"
+
+        aliases = lookup_any(current, ALIAS_MAPS)
+        if not aliases:
+            edges[current] = []
+            continue
+
+        targets = [
+            x.strip().lower()
+            for x in aliases.split(",")
+            if (
+                x.strip()
+                and x.strip().count("@") == 1
+                and x.strip().lower() != current
+            )
+        ]
+
+        edges[current] = targets
+
+        for target in targets:
+            if target == recipient:
+                if depth + 1 <= MAX_ALIAS_DEPTH:
+                    return True, "Alias-Kette erkannt"
+                depth_exceeded = True
+                continue
+
+            if target in visited:
+                continue
+
+            if depth + 1 >= MAX_ALIAS_DEPTH:
+                depth_exceeded = True
+                continue
+
+            visited.add(target)
+            pending.append((target, depth + 1))
+
+    if depth_exceeded:
+        return None, "Maximale Alias-Tiefe erreicht"
+
+    # Detect cycles in the explored directed graph.
+    # 0 = unseen, 1 = active, 2 = completed
+    state = {}
+
+    def has_cycle(node):
+        state[node] = 1
+
+        for target in edges.get(node, []):
+            if target not in edges:
+                continue
+
+            status = state.get(target, 0)
+
+            if status == 1:
+                return True
+
+            if status == 0 and has_cycle(target):
+                return True
+
+        state[node] = 2
+        return False
+
+    for node in edges:
+        if state.get(node, 0) == 0 and has_cycle(node):
+            return None, "Alias-Schleife erkannt"
+
+    return False, "Empfaenger nicht in direkter Alias-Kette"
+
+
+def decide(sender, recipient, original):
+    sender = sender.strip().lower()
+    recipient = recipient.strip().lower()
+    original = original.strip().lower()
+
+    if not sender or sender == "<>":
+        return "NO_SRS", "Null-Absender"
+
+    if sender.count("@") != 1 or recipient.count("@") != 1:
+        return "UNKNOWN", "Ungueltige Empfaengeradresse"
+
+    if original.count("@") == 0:
+        return "NO_SRS", "Lokaler Alias"
+
+    if original.count("@") != 1:
+        return "UNKNOWN", "Ungueltige Empfaengeradresse"
+
+    if recipient == original:
+        return "NO_SRS", "Keine Empfaengerumschreibung"
+
+    reaches, reason = alias_reaches(original, recipient)
+    if reaches is not True:
+        return "UNKNOWN", reason
+
+    destination = classify_destination(recipient)
+    if destination == "LOCAL":
+        return "NO_SRS", "Lokales Postfach"
+    if destination == "EXTERNAL":
+        return "SRS", "PostfixAdmin-Alias-Kette mit SMTP-Zustellung"
+    return "UNKNOWN", "Zustellweg nicht eindeutig"
+
+
+def main():
+    if len(sys.argv) != 4:
+        print(
+            "Aufruf: srs-alias-decision.py "
+            "<sender> <recipient> <original_recipient>",
+            file=sys.stderr,
+        )
+        return 2
+
+    try:
+        decision, reason = decide(*sys.argv[1:])
+    except (RuntimeError, subprocess.TimeoutExpired) as exc:
+        print(f"UNKNOWN: Lookup-Fehler: {exc}")
+        return 1
+
+    print(f"{decision}: {reason}")
+    return 1 if decision == "UNKNOWN" else 0
+
+
+if __name__ == "__main__":
+    sys.exit(main())
diff --git a/files/srs/srs-forward.py b/files/srs/srs-forward.py
new file mode 100755
index 0000000..278dcaf
--- /dev/null
+++ b/files/srs/srs-forward.py
@@ -0,0 +1,177 @@
+#!/usr/bin/python3
+
+import smtplib
+import subprocess
+import sys
+
+SMTP_HOST = "127.0.0.1"
+SMTP_PORT = 10028
+
+DECISION_SCRIPT = "/usr/local/sbin/srs-alias-decision.py"
+SRS_MAP = "socketmap:inet:127.0.0.1:10003:forward"
+
+EX_TEMPFAIL = 75
+EX_UNAVAILABLE = 69
+
+
+def run_command(args):
+    result = subprocess.run(
+        args,
+        capture_output=True,
+        text=True,
+        timeout=30
+    )
+
+    if result.returncode != 0:
+        raise RuntimeError(
+            result.stderr.strip()
+            or result.stdout.strip()
+            or f"Exit-Code {result.returncode}"
+        )
+
+    return result.stdout.strip()
+
+
+def determine_sender(sender, recipient, original_recipient):
+    decision = run_command([
+        DECISION_SCRIPT,
+        sender,
+        recipient,
+        original_recipient
+    ])
+
+    if decision.startswith("NO_SRS:"):
+        return sender
+
+    if not decision.startswith("SRS:"):
+        raise RuntimeError(
+            f"Keine eindeutige SRS-Entscheidung: {decision}"
+        )
+
+    rewritten_sender = run_command([
+        "/usr/sbin/postmap",
+        "-q",
+        sender,
+        SRS_MAP
+    ])
+
+    if not rewritten_sender:
+        raise RuntimeError("PostSRSd lieferte keinen Absender")
+
+    if not rewritten_sender.lower().endswith("@srs.warenform.de"):
+        raise RuntimeError(
+            f"Unerwartete SRS-Domain: {rewritten_sender}"
+        )
+
+    if not rewritten_sender.upper().startswith(("SRS0=", "SRS1=")):
+        raise RuntimeError(
+            f"Ungueltige SRS-Adresse: {rewritten_sender}"
+        )
+
+    return rewritten_sender
+
+
+def main():
+    if len(sys.argv) not in (5, 6):
+        print(
+            "Aufruf: srs-forward.py "
+            "sender recipient original_recipient queue_id [envid]",
+            file=sys.stderr
+        )
+        return EX_TEMPFAIL
+
+    sender, recipient, original_recipient, queue_id = sys.argv[1:5]
+    envid = sys.argv[5] if len(sys.argv) == 6 else ""
+
+    if envid.startswith("envid="):
+        envid = envid[len("envid="):]
+
+    try:
+        envelope_sender = determine_sender(
+            sender,
+            recipient,
+            original_recipient
+        )
+
+        message = sys.stdin.buffer.read()
+
+        with smtplib.SMTP(
+            SMTP_HOST,
+            SMTP_PORT,
+            timeout=30
+        ) as smtp:
+            smtp.ehlo()
+
+            mail_options = []
+            rcpt_options = []
+
+            if envid:
+                if not smtp.has_extn("dsn"):
+                    raise RuntimeError(
+                        "SMTP-Ziel unterstuetzt kein DSN/ENVID"
+                    )
+                mail_options.append(f"ENVID={envid}")
+
+            if original_recipient and smtp.has_extn("dsn"):
+                rcpt_options.append(
+                    f"ORCPT=rfc822;{original_recipient}"
+                )
+
+            smtp.sendmail(
+                envelope_sender,
+                [recipient],
+                message,
+                mail_options=mail_options,
+                rcpt_options=rcpt_options
+            )
+
+    except smtplib.SMTPRecipientsRefused as exc:
+        codes = [
+            code
+            for code, response in exc.recipients.values()
+        ]
+
+        permanent = bool(codes) and all(
+            500 <= code < 600 for code in codes
+        )
+
+        result = EX_UNAVAILABLE if permanent else EX_TEMPFAIL
+        enhanced_status = "5.0.0" if permanent else "4.0.0"
+
+        print(
+            f"{enhanced_status} SRS queue_id={queue_id}: "
+            f"SMTP-Empfaengerablehnung: {exc.recipients!r}",
+            file=sys.stderr
+        )
+        return result
+
+    except smtplib.SMTPResponseException as exc:
+        code = exc.smtp_code
+        result = EX_UNAVAILABLE if 500 <= code < 600 else EX_TEMPFAIL
+
+        enhanced_status = "5.0.0" if result == EX_UNAVAILABLE else "4.0.0"
+
+        print(
+            f"{enhanced_status} SRS queue_id={queue_id}: "
+            f"SMTP {code}: {exc.smtp_error!r}",
+            file=sys.stderr
+        )
+        return result
+
+    except (
+        subprocess.TimeoutExpired,
+        RuntimeError,
+        smtplib.SMTPException,
+        OSError
+    ) as exc:
+        print(
+            f"SRS queue_id={queue_id}: temporaerer Fehler: {exc}",
+            file=sys.stderr
+        )
+        return EX_TEMPFAIL
+
+    return 0
+
+
+if __name__ == "__main__":
+    sys.exit(main())
diff --git a/files/srs/srs-reverse-map.py b/files/srs/srs-reverse-map.py
new file mode 100755
index 0000000..4203f4c
--- /dev/null
+++ b/files/srs/srs-reverse-map.py
@@ -0,0 +1,112 @@
+#!/usr/bin/python3
+
+import socket
+import socketserver
+
+LISTEN_HOST = "127.0.0.1"
+LISTEN_PORT = 10004
+SRS_HOST = "127.0.0.1"
+SRS_PORT = 10003
+SRS_DOMAIN = "srs.warenform.de"
+
+MAX_REQUEST = 8192
+TIMEOUT = 3
+
+
+def read_netstring(sock):
+    length = bytearray()
+
+    while True:
+        c = sock.recv(1)
+        if not c:
+            raise EOFError()
+        if c == b":":
+            break
+        if not c.isdigit() or len(length) >= 8:
+            raise ValueError("Invalid netstring length")
+        length.extend(c)
+
+    if not length:
+        raise ValueError("Empty netstring length")
+
+    size = int(length)
+    if size > MAX_REQUEST:
+        raise ValueError("Netstring too large")
+
+    data = bytearray()
+    while len(data) < size:
+        chunk = sock.recv(size - len(data))
+        if not chunk:
+            raise EOFError()
+        data.extend(chunk)
+
+    if sock.recv(1) != b",":
+        raise ValueError("Invalid netstring terminator")
+
+    return bytes(data)
+
+
+def netstring(data):
+    return str(len(data)).encode() + b":" + data + b","
+
+
+class Handler(socketserver.BaseRequestHandler):
+    def handle(self):
+        self.request.settimeout(TIMEOUT)
+
+        while True:
+            try:
+                request = read_netstring(self.request)
+            except (EOFError, OSError):
+                return
+            except ValueError:
+                self.request.sendall(netstring(b"PERM Invalid request"))
+                return
+
+            try:
+                operation, address = request.decode("utf-8").split(" ", 1)
+            except (UnicodeError, ValueError):
+                self.request.sendall(netstring(b"PERM Invalid request"))
+                continue
+
+            if operation != "reverse":
+                self.request.sendall(netstring(b"NOTFOUND "))
+                continue
+
+            if "@" not in address:
+                self.request.sendall(netstring(b"NOTFOUND "))
+                continue
+
+            domain = address.rsplit("@", 1)[1].lower()
+
+            if domain != SRS_DOMAIN:
+                self.request.sendall(netstring(b"NOTFOUND "))
+                continue
+
+            try:
+                with socket.create_connection(
+                    (SRS_HOST, SRS_PORT), timeout=TIMEOUT
+                ) as upstream:
+                    upstream.settimeout(TIMEOUT)
+                    upstream.sendall(netstring(request))
+                    response = read_netstring(upstream)
+
+                if response.startswith((b"OK ", b"NOTFOUND ", b"TEMP ", b"PERM ")):
+                    result = response
+                else:
+                    result = b"TEMP Invalid upstream response"
+
+            except (OSError, EOFError, ValueError):
+                result = b"TEMP SRS backend unavailable"
+
+            self.request.sendall(netstring(result))
+
+
+class Server(socketserver.ThreadingTCPServer):
+    allow_reuse_address = True
+    daemon_threads = True
+
+
+if __name__ == "__main__":
+    with Server((LISTEN_HOST, LISTEN_PORT), Handler) as server:
+        server.serve_forever()
diff --git a/files/srs/srs-reverse-policy.py b/files/srs/srs-reverse-policy.py
new file mode 100755
index 0000000..30066c3
--- /dev/null
+++ b/files/srs/srs-reverse-policy.py
@@ -0,0 +1,113 @@
+#!/usr/bin/env python3
+
+import socket
+import sys
+
+SRS_DOMAIN = "srs.warenform.de"
+SOCKET_HOST = "127.0.0.1"
+SOCKET_PORT = 10003
+TIMEOUT = 5
+MAX_RESPONSE = 4096
+
+
+def read_netstring(sock):
+    length_bytes = bytearray()
+
+    while True:
+        char = sock.recv(1)
+        if not char:
+            raise ValueError("Incomplete netstring length")
+        if char == b":":
+            break
+        if char < b"0" or char > b"9" or len(length_bytes) >= 8:
+            raise ValueError("Invalid netstring length")
+        length_bytes.extend(char)
+
+    if not length_bytes:
+        raise ValueError("Empty netstring length")
+
+    length = int(length_bytes)
+
+    if length > MAX_RESPONSE:
+        raise ValueError("Netstring response too large")
+
+    data = bytearray()
+
+    while len(data) < length + 1:
+        chunk = sock.recv(length + 1 - len(data))
+        if not chunk:
+            raise ValueError("Incomplete netstring response")
+        data.extend(chunk)
+
+    if data[-1:] != b",":
+        raise ValueError("Invalid netstring terminator")
+
+    return bytes(data[:-1]).decode("utf-8")
+
+
+def verify_srs(address):
+    request = f"reverse {address}".encode("utf-8")
+    packet = str(len(request)).encode("ascii") + b":" + request + b","
+
+    with socket.create_connection(
+        (SOCKET_HOST, SOCKET_PORT), timeout=TIMEOUT
+    ) as sock:
+        sock.settimeout(TIMEOUT)
+        sock.sendall(packet)
+        response = read_netstring(sock)
+
+    if response.startswith("OK "):
+        original = response[3:]
+        if not original or "@" not in original:
+            raise ValueError("Invalid reverse result")
+        return "OK"
+
+    if response.startswith("NOTFOUND "):
+        return "REJECT 550 5.1.1 Invalid SRS address"
+
+    raise ValueError(f"Unexpected socketmap response: {response}")
+
+
+def check_recipient(recipient):
+    if "@" not in recipient:
+        return "DUNNO"
+
+    localpart, domain = recipient.rsplit("@", 1)
+
+    if domain.lower() != SRS_DOMAIN:
+        return "DUNNO"
+
+    if not localpart.upper().startswith(("SRS0=", "SRS1=")):
+        return "REJECT 550 5.1.1 Invalid SRS address"
+
+    try:
+        return verify_srs(recipient)
+    except (OSError, UnicodeError, ValueError) as exc:
+        print(f"SRS verification error: {exc}", file=sys.stderr)
+        return "DEFER 451 4.3.0 SRS verification unavailable"
+
+
+def main():
+    while True:
+        attributes = {}
+
+        for line in sys.stdin:
+            line = line.rstrip("\r\n")
+
+            if not line:
+                break
+
+            if "=" in line:
+                key, value = line.split("=", 1)
+                attributes[key] = value
+        else:
+            return
+
+        recipient = attributes.get("recipient", "")
+        action = check_recipient(recipient)
+
+        print(f"action={action}\n", flush=True)
+
+
+if __name__ == "__main__":
+    main()