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Opens one SMTP session per domain (not per address), probes a high-entropy
gibberish address once to learn whether the domain is catch-all, caches that
domain-level fact, and only then probes the real mailboxes.
Why this matters:
- A TCP + EHLO handshake dominates the cost of a probe, so reusing one session
for many RCPTs is the difference between a 10k list taking minutes vs hours.
- Catch-all is a *domain* property, so probing it once per domain (and caching
it for 24-72h) removes a redundant probe for every other address there.
- On a known catch-all domain no per-address RCPT can distinguish a real
mailbox from a fake one, so the honest answer is "unverifiable" — returned
without burning a connection.
Every remote failure is data, never an exception: one dead host cannot abort a
bulk job.
"""
from __future__ import annotations
import asyncio
import random
import time
from dataclasses import dataclass
from typing import Callable, Iterable, Optional
from ..models import SMTPResult
from .mx_intel import classify_mx
from .smtp_client import RcptOutcome, SmtpConnection
DEFAULT_CATCH_ALL_TTL = 72 * 3600 # 72h — catch-all status is stable
@dataclass
class CatchAllCache:
"""Per-domain catch-all memory.
`None` means "probe could not decide" and is deliberately NOT cached — a
failed probe must not be remembered as a decision in either direction.
"""
ttl_seconds: int = DEFAULT_CATCH_ALL_TTL
def __post_init__(self) -> None:
self._store: dict[str, tuple[bool, float]] = {}
def put(self, domain: str, value: Optional[bool]) -> None:
if value is None:
return
self._store[domain.lower()] = (bool(value), time.time() + self.ttl_seconds)
def get(self, domain: str) -> Optional[bool]:
item = self._store.get(domain.lower())
if item is None:
return None
value, expires = item
if time.time() >= expires:
self._store.pop(domain.lower(), None)
return None
return value
def clear(self) -> None:
self._store.clear()
def _gibberish(domain: str) -> str:
token = f"{random.getrandbits(48):012x}"
return f"xq{token}@{domain}"
class MxBatchVerifier:
"""Verify many (email, mx_host) pairs with domain-level session reuse."""
def __init__(
self,
connection_factory: Optional[Callable[..., object]] = None,
*,
timeout: float = 10.0,
helo_domain: str = "validator.local",
mail_from: str = "probe@validator.local",
catch_all_cache: Optional[CatchAllCache] = None,
max_connections_per_host: int = 2,
fail_threshold: int = 3,
cooldown_seconds: int = 300,
bulk_concurrency: int = 20,
):
self.timeout = timeout
self.helo_domain = helo_domain
self.mail_from = mail_from
self.catch_all_cache = catch_all_cache or CatchAllCache()
self.max_connections_per_host = max_connections_per_host
self._factory = connection_factory or (
lambda host, **kw: SmtpConnection(host, **kw)
)
# Egress circuit breaker. On a host that blocks outbound port 25 every
# connect burns the full timeout, so a 10k job would take ~27h. After
# `fail_threshold` consecutive connect failures we stop dialling for
# `cooldown_seconds` and report rows unavailable immediately instead.
self.fail_threshold = max(1, fail_threshold)
self.cooldown_seconds = cooldown_seconds
self.bulk_concurrency = max(1, bulk_concurrency)
self._consecutive_failures = 0
self._open_until = 0.0
@property
def circuit_open(self) -> bool:
return time.monotonic() < self._open_until
def _record_connect_failure(self) -> None:
self._consecutive_failures += 1
if self._consecutive_failures >= self.fail_threshold:
self._open_until = time.monotonic() + self.cooldown_seconds
def _record_connect_success(self) -> None:
self._consecutive_failures = 0
self._open_until = 0.0
def _new_connection(self, mx_host: str):
return self._factory(
mx_host,
timeout=self.timeout,
helo_domain=self.helo_domain,
mail_from=self.mail_from,
)
def verify_batch(
self, pairs: Iterable[tuple[str, str]]
) -> dict[str, SMTPResult]:
"""Verify pairs of (email, mx_host), sequentially. Returns email -> result."""
groups = self._group(pairs)
results: dict[str, SMTPResult] = {}
for (domain, mx_host), emails in groups.items():
results.update(self._verify_domain(domain, mx_host, emails))
return results
async def verify_batch_async(
self, pairs: Iterable[tuple[str, str]]
) -> dict[str, SMTPResult]:
"""Verify many pairs with DOMAINS PROBED CONCURRENTLY.
Blocking SMTP work runs in a thread pool bounded by `bulk_concurrency`.
Sequential probing makes wall-clock the sum of every domain's latency
(timeout x domains on a blocked host); real bulk throughput comes from
parallelism across domains, not speed per domain.
"""
groups = self._group(pairs)
if not groups:
return {}
semaphore = asyncio.Semaphore(self.bulk_concurrency)
async def run_domain(domain: str, mx_host: str, emails: list[str]):
async with semaphore:
return await asyncio.to_thread(
self._verify_domain, domain, mx_host, emails
)
chunks = await asyncio.gather(
*(run_domain(d, m, e) for (d, m), e in groups.items()),
return_exceptions=True,
)
results: dict[str, SMTPResult] = {}
for chunk in chunks:
if isinstance(chunk, Exception):
continue
results.update(chunk)
# Any group that raised must still return a row per address.
missing = [
(d, m, e)
for (d, m), e in groups.items()
if not any(addr in results for addr in e)
]
for domain, mx_host, emails in missing:
for email in emails:
results.setdefault(
email,
SMTPResult(
verified=None,
error=f"smtp_unreachable: {mx_host} (probe aborted)",
),
)
return results
@staticmethod
def _group(
pairs: Iterable[tuple[str, str]]
) -> dict[tuple[str, str], list[str]]:
"""Group (email, mx_host) pairs by (domain, mx) — the session unit."""
groups: dict[tuple[str, str], list[str]] = {}
for email, mx_host in pairs:
if not email or not mx_host:
continue
domain = email.rsplit("@", 1)[-1].lower()
groups.setdefault((domain, mx_host), []).append(email)
return groups
def _verify_domain(
self, domain: str, mx_host: str, emails: list[str]
) -> dict[str, SMTPResult]:
cached = self.catch_all_cache.get(domain)
# Known catch-all: no per-address probe can confirm a mailbox here.
if cached is True:
return {
e: SMTPResult(verified=None, error=f"catch_all (cached): {domain}")
for e in emails
}
out: dict[str, SMTPResult] = {}
# Fail fast while egress is known-bad: do not dial, do not wait.
if self.circuit_open:
return {
e: SMTPResult(
verified=None,
error=f"smtp_unreachable: {mx_host} (egress circuit open)",
)
for e in emails
}
conn = self._new_connection(mx_host)
try:
if not conn.open():
self._record_connect_failure()
return {
e: SMTPResult(
verified=None,
error=f"smtp_unreachable: {mx_host}",
)
for e in emails
}
self._record_connect_success()
if cached is None:
ca = conn.probe(_gibberish(domain))
if ca.accepted:
self.catch_all_cache.put(domain, True)
for e in emails:
out[e] = SMTPResult(
verified=None,
error=f"catch_all: {domain} accepts every recipient",
)
return out
if ca.permanent_reject:
self.catch_all_cache.put(domain, False)
# else inconclusive -> do not cache, fall through and probe.
policy = classify_mx(mx_host)
for email in emails:
out[email] = self._interpret(conn.probe(email), policy, domain)
except Exception as exc: # defensive: never let one group kill the job
for email in emails:
out.setdefault(
email, SMTPResult(verified=None, error=f"probe_error: {exc}")
)
finally:
try:
conn.quit()
except Exception:
pass
return out
@staticmethod
def _interpret(outcome: RcptOutcome, policy, domain: str) -> SMTPResult:
if outcome.transport_error:
return SMTPResult(
verified=None, error=f"transport_error: {outcome.error or 'unknown'}"
)
if outcome.accepted:
if policy.accepts_all:
# Microsoft 365 / gateways accept everything and bounce later —
# a 250 here is not proof of a mailbox.
return SMTPResult(
verified=None,
error=f"accept_all_provider ({policy.provider}): rcpt not conclusive",
)
return SMTPResult(
verified=True, smtp_code=250, smtp_message=outcome.message[:400]
)
if outcome.permanent_reject:
return SMTPResult(
verified=False,
smtp_code=outcome.code,
smtp_message=outcome.message[:400],
)
# 4xx greylist or unexpected code: inconclusive, retryable.
return SMTPResult(
verified=None,
smtp_code=outcome.code,
error=f"inconclusive: {outcome.message[:200] or outcome.code}",
) |