Azure SMTP relay: Azure Communication Services, smtp.azurecomm.net, port 587
Azure has no generic SMTP relay; Azure Communication Services Email provides one: host smtp.azurecomm.net, port 587 STARTTLS, username = "<ACS resource>.<Entra app id>.<tenant id>" and an app client secret. Also: Azure VMs block port 25, and the SendGrid/Office 365 alternatives.
Azure Communication Services outgoing mail server: smtp.azurecomm.net.
Azure Communication Services SMTP settings at a glance
| Outgoing (SMTP) host | smtp.azurecomm.net |
|---|---|
| Ports & encryption |
587 — STARTTLS <em>(the only port)</em> |
| Username | the SMTP username of the form <ACS-resource-name>.<Entra-application-id>.<tenant-id> (shown after creating SMTP credentials in the ACS resource), or the plain Entra app details as documented |
| Password | the Microsoft Entra application's client secret |
| Authentication | Create an Entra app registration, grant it a role on the Communication Services resource, then in the ACS resource → Email → SMTP Usernames create the username. Office 365 users can instead use smtp.office365.com; Azure VMs cannot send on port 25 outbound (blocked on most subscriptions). |
How to set up Azure Communication Services SMTP
- Create Communication Services + Email Communication Services resources; add and verify a domain; connect the domain to ACS.
- Entra → App registrations → new app → client secret; assign the app a Communication and Email Service Owner role on the ACS resource.
- ACS resource → Email → SMTP Usernames → create username linking the app.
- App settings: host smtp.azurecomm.net, port 587, STARTTLS, username = SMTP username, password = client secret.
Sending limits
- ACS Email charges per message and per data; new resources have low per-minute/hour quotas that can be raised via support.
- A verified custom domain (SPF, DKIM) or an Azure-managed domain is required.
Things that catch people out
- Port 25 outbound from Azure VMs is blocked unless the subscription is exempted — sending servers must relay on 587.
Common Azure Communication Services SMTP errors
| Server reply | Meaning | Fix |
|---|---|---|
535 5.7.3 Authentication unsuccessful | Role or username format wrong. | Check the app role assignment and the generated SMTP username. |
More codes: 535 authentication failed, 550 relay denied, 554 rejected as spam, connection timed out — or all SMTP error codes.
When Azure Communication Services is not enough for your app
When a provider caps you or blocks a port, point the same app at QueenSMTP instead: host smtp.queensmtp.com, port 587 (STARTTLS) or 465 (SSL), your SMTP username and password from the dashboard. $5/year plus $0.10 per 1,000 emails, 100 free a day to test.
Get SMTP credentials — free to test Test your relay nowRelated settings
Azure Communication Services SMTP — questions
Not a generic one. Azure Communication Services Email exposes smtp.azurecomm.net on port 587 with Entra-app credentials; Microsoft 365 users can use smtp.office365.com; otherwise use a third-party relay, because Azure VMs block outbound port 25.
Related Guides
Continue learning with these related articles