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Messaging Channels Overview

Azure Communication Services (ACS) provides multiple channels for asynchronous and real-time messaging. Each channel is designed for specific use cases, ranging from high-volume transactional notifications to interactive customer support chats.

Messaging Channels Comparison

Feature SMS Email Chat
Primary Use Case Urgent alerts, MFA, marketing Invoices, reports, newsletters Customer support, internal collaboration
Persistence Device-dependent Inbox-dependent Server-side (stored in ACS)
Real-time Near real-time Low-to-moderate latency Instant (via WebSockets)
Formatting Plain text (mostly) Rich HTML / Attachments Plain text, Emoji, Attachments
Capacity High volume (Short codes) Very high (Transactional) Threads / Large groups

SMS Channel

ACS allows you to send and receive text messages globally. To use the SMS channel, you must acquire a phone number within your Communication Resource.

Key Capabilities

  • Toll-Free Numbers: Ideal for high-volume messages and two-way communication.
  • Short Codes: Specialized numbers for massive transactional or promotional campaigns.
  • MMS Support: Send and receive multimedia content (images, video).
  • Delivery Status: Real-time tracking of sent messages via Event Grid.

Email Channel

The ACS Email channel provides a high-reliability platform for outbound (application-to-person) transactional email. Microsoft Learn documents only sending capabilities for ACS Email — SendMail plus delivery and engagement tracking — so if your domain must also receive replies, keep the MX record pointed at your separate inbound mail provider (Microsoft 365, Google Workspace, your own MTA, etc.) and use ACS only for the outbound leg.

Key Capabilities

  • Azure Managed Domains: Instant setup using donotreply@xxxx.azurecomm.net.
  • Custom Domains: Verified domains using four Portal DNS wizards (Domain TXT, SPF, DKIM, DKIM2) plus an optional DMARC TXT for high deliverability. No ACS-specific MX record is needed — the documented DNS setup covers only outbound sending. See Email provisioning for the full DNS setup.
  • Batch Sending: API-optimized for sending messages to thousands of recipients.
  • Tracking: Support for delivery, bounce, and click tracking.

Chat Channel

The Chat channel enables real-time messaging between users in chat "threads".

Key Capabilities

  • Thread Management: Create, update, and delete threads with up to 250 participants.
  • Real-time Notifications: Uses WebSockets to deliver messages instantly to active clients.
  • Read Receipts & Typing Indicators: Standard modern chat features built-in.
  • History Retrieval: Access to previous messages within a thread.

Messaging Architecture Diagram

The following diagram illustrates how your application interacts with different messaging channels via ACS.

graph TD
    App[Application Server] -- "REST / SDK" --> ACS[Azure Communication Services]

    ACS -- "Twilio/Telco" --> SMS[SMS Channel]
    ACS -- "MTA / SMTP" --> Email[Email Channel]
    ACS -- "WebSockets" --> Chat[Chat Channel]

    SMS --> Mobile[Mobile User]
    Email --> Inbox[Email User]
    Chat --> Browser[Web/Mobile App]

    ACS -- "Events" --> EventGrid[Azure Event Grid]
    EventGrid -.-> Callback[App Callback / Log]

Hybrid Messaging

Many applications use a hybrid approach: sending a real-time Chat message first and falling back to SMS if the user is offline for more than 5 minutes (via Event Grid monitoring).

See Also

Sources