レガシーとモダンを 1 つの信頼できるリレーに
多くのアプリ、プラグイン、社内サービスにとって SMTP は依然として最短ルートです。Sendarix は本番品質の SMTP 処理と明確なログ、イベント結果を提供します。
トランザクショントラフィック、通知ワークロード、プラットフォーム生成メッセージを同じリレーで扱い、壊れやすいワンオフツールを増やしません。
SMTP sessions flow through authentication, TLS, and routing enforcement before acceptance. The relay handles throttling, deferrals, and failover while delivery events are tracked via email webhooks and email analytics.
実運用の SMTP のために設計
単純な転送層ではありません。認証サポート、キュー規律、運用チーム向けの可観測性を備えた制御された送信パイプラインです。
安全な送信
SMTP AUTH と TLS 対応の送信フローで認証情報と転送を保護。
スループットの安定性
バーストトラフィックと日次のバックグラウンドメールに一貫したパフォーマンス。
可視性とログ
検索可能なログとイベントレベルの追跡で配信結果を迅速に調査。
スムーズな移行
旧 SMTP プロバイダーから、すぐに全体を書き換えずに移行。
専用の送信アイデンティティ
認証済みドメインとエンタープライズメール要件に沿った送信者ポリシーを使用。
キューを意識した信頼性
スパイク時も制御されたキューイングとリトライで配信の一貫性を維持。
SMTP リレーの流れ
インフラを予測可能かつデバッグしやすくする 4 ステップの流れ。
1. 接続
アプリケーションが SMTP 認証情報で接続し、安全なセッションを確立。
2. 受理
メッセージは検証され、制御された処理のためにキューに入ります。
3. ルーティング
トラフィックは回復力とプロバイダーに配慮した動作で配信パスを通ります。
4. 追跡
結果イベントとログが運用上の確信と迅速なトラブルシューティングを可能にします。
Real-World Scenario: Legacy System Integration
An internal system sends scheduled reports via SMTP. The relay authenticates with credentials, processes the message through spam filtering, and delivers it via email routing with TLS enforcement. If the destination server is temporarily unreachable, the retry logic queues the message and delivers it on recovery, with status tracked via email webhooks.
SMTP Command Flow
Understanding the actual SMTP protocol exchange helps when debugging delivery issues and interpreting bounce codes. Below is a complete example of a typical message submission session.
Example SMTP Session
This shows a full session from connection through message acceptance. Replace example.com with your verified sending domain.
S: 220 mail.sendarix.com ESMTP ready
C: EHLO mail.yourapp.com
S: 250-mail.sendarix.com
250-PIPELINING
250-SIZE 52428800
250-8BITMIME
250-STARTTLS
250-AUTH PLAIN LOGIN
250 CHUNKING
C: AUTH PLAIN dXNlcm5hbWUAdXNlcm5hbWUAYWQhZGFzZDhzNG
S: 235 2.7.0 Authentication successful
C: MAIL FROM:<notifications@yoursenddomain.com>
S: 250 2.1.0 OK
C: RCPT TO:<user@example.com>
S: 250 2.1.5 OK
C: DATA
S: 354 Start mail input; end with CRLF.CRLF
C: (message headers and body)
C: .
S: 250 2.0.0 OK: queued as abc123
C: QUIT
S: 221 2.0.0 Goodbye
SMTP Response Code Reference
220
— Service ready (greeting)
235
— Auth successful
250
— Message accepted
354
— Ready for DATA
421
— Try again later
450
— Mailbox temp unavailable
451
— Local error
452
— Storage full
550
— User unknown (hard bounce)
552
— Message too large
554
— Transaction failed (spam/policy)
Authentication and TLS Configuration
Sendarix supports both SMTP AUTH for credential-based submission and TLS for transport encryption. Proper configuration is required for reliable delivery and to avoid intermediate network interception.
SMTP AUTH Mechanisms
SMTP AUTH has several mechanisms. PLAIN and LOGIN are the most widely supported:
AUTH PLAIN
Sends credentials as a single base64-encoded string: [authzid]\0username\0password. Safe over TLS. Do not use over plain HTTP.
AUTH LOGIN
Two-step challenge-response: username prompt then password prompt. Base64-encoded at each step. Also requires TLS.
AUTH XOAUTH2 (OAuth 2.0)
OAuth 2.0 based SASL mechanism for applications using OAuth tokens instead of passwords. Preferred for SaaS platforms where users authorize sending on their own domains.
Recommended Configuration
For most applications: use port 587 with STARTTLS and AUTH PLAIN. Reserve port 465 for legacy clients that cannot upgrade. Never transmit AUTH credentials on plaintext connections.
TLS Configuration
smtp_relay.tls_opportunistic_h4
Connection starts unencrypted on port 587, then upgrades via STARTTLS command after EHLO. If the remote server does not advertise STARTTLS, the connection is rejected — Sendarix never sends credentials over plaintext.
Implicit TLS (SMTPS on 465)
Connection is encrypted from the TCP handshake. No STARTTLS upgrade needed. Preferred by enterprise clients and some legacy systems. Sendarix supports TLS 1.2 and 1.3 with modern cipher suites.
Certificate Verification
Sendarix presents a certificate signed by a known CA. Your SMTP client should verify the certificate chain against trusted root CAs. Disable certificate verification only in controlled test environments.
SMTP TLS Pinning
For high-security applications, you can pin the Sendarix leaf certificate. Monitor certificate rotation announcements and update pinned certificates before expiration.
Provider-Specific Behavior: Gmail vs Outlook
Gmail and Outlook (Microsoft) handle SMTP submission, rate limits, and spam filtering differently. Understanding these differences helps you tune your integration and interpret delivery failures accurately.
Gmail (Google Workspace / Gmail)
Authentication Requirements
Gmail requires SPF, DKIM, and DMARC all aligned for reliable inbox delivery. Messages failing DKIM signing or showing alignment failures are routed to spam or rejected. Sendarix DKIM signs all outbound mail from verified domains.
Rate Limits
Gmail enforces per-IP and per-user sending limits. Standard limits: 500 emails/recipient/day, 2000 emails/day per account for consumer Gmail, 2000 emails/minute for Workspace accounts. Exceeding limits returns 421 or 550 errors temporarily.
TLS Requirements
Gmail requires TLS for all inbound connections. Sending plaintext to Gmail is rejected at the MTA level. Sendarix enforces TLS for all major providers.
Bounce Behavior
Gmail returns 550 for hard bounces (invalid user) and 421 with retry for temporary issues. Soft bounces that persist across multiple delivery attempts convert to hard bounces internally and are suppressed from future sends.
Spam Filtering Signals
Gmail's ML-based spam filter evaluates sender history, engagement rates, content patterns, and authentication status. Low engagement (many marked as spam) degrades future delivery. Sending to engaged recipients consistently improves inbox placement.
Microsoft Outlook / Microsoft 365
Authentication Requirements
Microsoft requires SPF, DKIM (Microsoft-specific selector), and DMARC alignment. They also evaluate sender reputation via postmaster services and Smart Network Services (sns.oregon.trafficmanager.net). Without proper authentication, messages are filtered or rejected.
Rate Limits
Microsoft enforces limits per tenant and per IP: 30,000 emails/day for standard tenants, higher for premium. Per-minute limits vary by tenant age and reputation. Exceeding limits returns 421 and queues messages for retry.
TLS Requirements
Microsoft requires TLS 1.2+ for all inbound mail. They support TLS 1.3 and prefer it. Plaintext connections are rejected. Microsoft also enforces certificate validation — self-signed certs will fail.
Bounce Behavior
Microsoft uses enhanced bounce codes: 550 5.1.1 (unknown user), 550 5.1.2 (bad destination), 550 5.2.1 (mailbox unavailable). They also use 250 OK with diagnostic codes in the message headers for soft failures.
Complaint Handling
Microsoft reports complaints via Feedback Loop (FBL) integration. Messages marked as spam by recipients trigger complaint events in Sendarix. High complaint rates trigger Microsoft's "data fix" process, temporarily blocking your IP until reputation is restored.
Junk Folder Placement
Outlook uses Exchange Online Protection (EOP) with multiple filtering layers: connection filtering, policy filtering, and content filtering. Sending volume patterns, list hygiene, and engagement history all influence junk folder placement.
SMTP Relay Failover Architecture
Sendarix implements multi-layer failover to ensure message acceptance even during infrastructure and provider-level outages. The failover hierarchy operates automatically with no manual intervention required.
Failover Levels
Connection-Level Failover
When the primary SMTP endpoint returns connection errors, timeouts, or 421 retry responses, traffic immediately routes to the next available endpoint in the pool. Connection failures are detected within 5 seconds and failover initiates within 10 seconds.
IP-Level Failover
If a specific sending IP is rate-limited or reputation-blocked at a destination provider, that IP is automatically deprioritized and traffic shifts to alternative IPs in the same reputation tier. Warmup status is respected — cold IPs do not receive production traffic during failover.
Provider-Level Failover
When a destination provider (e.g., Gmail API ingestion) is unavailable, messages queue locally and retry at increasing intervals. The queue is durable — messages are not lost during extended provider outages. Queue depth is visible in the Sendarix dashboard.
Region-Level Failover
For enterprise configurations with multi-region sending, regional endpoints in different data centers handle traffic independently. DNS-based routing directs submission to the nearest available region.
DNS-Based Failover
MX records for your sending domain point to clustered ingress nodes. Sendarix manages DNS failover automatically — no manual DNS updates required when a node fails. TTL is set to 60 seconds to allow fast failover while maintaining stability.
Testing Failover Behavior
Sendarix provides sandbox endpoints for testing failover without affecting production reputation. Use the sandbox to simulate connection failures, 421 responses, and timeout conditions to verify your application handles relay failover gracefully.
Common SMTP Relay Use Cases
SMTP relay is the right choice when you need broad compatibility, existing tool integration, or transactional workload handling without API changes.
Legacy Application Integration
Applications built before modern email APIs existed often have built-in SMTP configuration. Sendarix relay replaces old provider credentials without requiring code changes. Update SMTP host, port, username, and password to start sending.
WordPress / CMS Notification Mail
WordPress, Drupal, and other CMS platforms send email via the server's configured SMTP relay. Installing an SMTP plugin and pointing it to Sendarix routes all notification, registration, and contact form emails through production infrastructure.
CI/CD Pipeline Notifications
Jenkins, GitLab CI, GitHub Actions, and similar tools send build status, deployment alerts, and run reports via SMTP. Configure the CI tool's SMTP settings to Sendarix to route these reliably and avoid alerts going to spam.
Internal Service Alerts
Monitoring tools (Datadog, PagerDuty, Prometheus Alertmanager), cron job failures, and system health checks often rely on email for critical notifications. Sendarix relay ensures these important alerts reach the on-call engineer's inbox.
SaaS Multi-Tenant Notification Sending
SaaS platforms sending notifications on behalf of their customers (appointment reminders, order updates, account alerts) can segment traffic by customer tenant using subaddresses or header-based routing. Each tenant gets isolated reputation management.
E-Commerce Order Confirmations
Order confirmation, shipping update, and delivery notification emails are critical transactional messages that drive customer engagement. SMTP relay integration with e-commerce platforms (Shopify, WooCommerce, Magento) ensures these emails land in the inbox reliably.
Ready to Route Email Through Production Infrastructure?
Get SMTP credentials and start sending in minutes. No architecture changes required.
What sets Sendarix apart: Sendarix SMTP relay runs on the same routing infrastructure as the email API — meaning you get failover, routing rules, and analytics without choosing between SMTP compatibility and modern email infrastructure control.
Related SMTP Configuration Guides
Step-by-step SMTP configuration for the most common email providers and platforms. Each guide covers host, port, encryption, authentication, and provider-specific behavior.
Gmail SMTP Settings
smtp.gmail.com · port 587 · TLS · App passwords
Outlook SMTP Configuration
smtp-mail.outlook.com · port 587 · TLS · OAuth
Yahoo SMTP Settings
smtp.mail.yahoo.com · port 587 · TLS · App password
Office 365 SMTP Setup
smtp.office365.com · port 587 · TLS · OAuth2
Zoho Mail SMTP Settings
smtp.zoho.com · port 587 · TLS · SMTP auth
iCloud SMTP Configuration
smtp.mail.me.com · port 587 · TLS · App password
Proton Mail SMTP Settings
smtp.protonmail.com · port 587 · TLS · Bridge
Yandex SMTP Configuration
smtp.yandex.com · port 587 · TLS · SMTP pass
GMX SMTP Settings
smtp.gmx.com · port 587 · TLS · App password
FastMail SMTP Configuration
smtp.fastmail.com · port 587 · TLS · App password
SendGrid SMTP Settings
smtp.sendgrid.net · port 587 · TLS · API key
Mailgun SMTP Configuration
smtp.mailgun.org · port 587 · TLS · SMTP credentials
Postmark SMTP Settings
smtp.postmarkapp.com · port 587 · TLS · Server token
Amazon SES SMTP Configuration
email-smtp.*.amazonaws.com · port 587 · TLS · SMTP credentials
Mailchimp Mandrill SMTP Settings
smtp.mandrillapp.com · port 587 · TLS · API key
All SMTP configuration guides include provider-specific details for authentication methods, rate limits, TLS requirements, and bounce handling behavior. These guides complement the email routing controls available in your Sendarix dashboard.
よくある質問
SMTP から始めて後から API に移れますか？
はい。多くのチームは速度のために SMTP から始め、より深いプロダクトワークフロー用に API を追加します。
配信レベルの可視性はありますか？
はい。ログとイベントストリームで受理、配信、バウンスの結果を確認できます。
高ボリューム送信に適していますか？
はい。リレーは本番プラットフォームの持続トラフィックとバースト向けに構築されています。
どのポートと暗号化を使うべきですか？
多くの本番クライアントは 587 で STARTTLS、または 465 で暗黙の TLS を使用します。公開ネットワークでは平文 SMTP を避けてください。
SMTP を既知の IP に限定できますか？
はい。IP 許可リストは一般的なエンタープライズ制御です。アプリサーバー、VPN 出口、または MTA フリートのみがリレーに認証できます。
アプリで認証情報はどう扱うべきですか？
SMTP 認証情報はシークレットマネージャーに保管し、人事異動時にローテーションし、ソースにコミットしないでください。環境ごとの認証情報を推奨します。
メールごとに新しい SMTP 接続を開くと配信に悪影響がありますか？
過度な接続の乱れは遅延と負荷を増やします。スタックが許す限り接続を責任を持って再利用し、文書化されたレートと同時実行のガイダンスに従ってください。
レガシー CRM やプリンターは同じ SMTP リレーを使えますか？
多くの場合は可能です。SMTP は旧システムの共通項です。プランがサポートする場合はドメイン、ヘッダー、またはサブアカウントでストリームを分離できます。
メッセージサイズと添付の制限は？
大きなペイロードは失敗リスクと処理時間を増やします。プランの制限を確認し、添付が重い場合は外部ホストにリンクを貼ることを検討してください。
What is the difference between port 587 and port 465?
Port 587 uses STARTTLS (opportunistic TLS upgrade after CONNECT), while port 465 uses implicit TLS where the connection is encrypted from the start. Both are valid for secure submission. Port 587 is the RFC-compliant standard for message submission; port 465 is deprecated but still widely supported for legacy clients.
How does Sendarix handle greylisting?
When a destination provider returns a 450 or 451 temporary response, Sendarix respects the retry period specified in the SMTP response. Most providers implement greylisting for 30 seconds to 10 minutes. Sendarix tracks these windows and retries within the provider's specified interval to maximize first-attempt success.
What is the maximum message size via SMTP?
Default maximum message size is 50MB including attachments and MIME encoding. For files larger than 25MB, Sendarix recommends uploading to cloud storage and sending a signed link instead. Some providers reject messages over 25MB entirely, regardless of SMTP acceptance.
Can we use subaddressing (user+tag@example.com) with Sendarix?
Yes. Subaddressing is fully supported and passed through to destination mail servers. The plus-address portion is visible to receiving MTA filters and can be used for routing, tagging, or filtering rules on the recipient side.
How do we verify our sending domain with Sendarix?
Domain verification requires adding DKIM records to your DNS. Sendarix provides a DKIM public key pair during setup. Add the TXT record to your domain's DNS, then Sendarix validates the record propagates before signing outgoing mail on your behalf.
