Unable to create SMTP usernames on Azure Communication Service — adding returns 409 "The username is not available" despite the resource not existing.

Ivan Svoboda 20 Reputation points
2026-04-14T10:37:10.83+00:00

I had an existing Azure Communication Service instance with two domains configured, each with several SMTP usernames. I deleted the resource group containing the ACS instance and all its resources, and then created a brand new ACS instance in a new resource group under the same subscription, with the same two domains.

When attempting to recreate the same SMTP usernames on the new ACS instance, I consistently receive a 409 Conflict error despite the username not existing anywhere:

RESPONSE 409: 409 Conflict

ERROR CODE: ResourcePut Failed

{

"error": {

"code": "ResourcePut Failed",

"message": "The username is not available."

}

}Screenshot 2026-04-14 at 12.28.40

Steps taken to investigate

  • GET on the specific SMTP username resource returns 404 — the resource does not exist on the new ACS instance
  • GET on the smtpUsernames collection on the new ACS instance returns an empty array { "value": [] }
  • PUT with a username string that did NOT previously exist on the old ACS succeeds without issue
  • PUT fails only for username strings that existed on the old ACS instance
  • The Entra App Registrations used in the request are newly created — they are not the same ones used previously
  • I have verified there are no remaining ACS resources in either subscription
Azure Communication Services
Answer accepted by question author
Praneeth Maddali 12,670 Reputation points Microsoft External Staff Moderator
2026-05-20T22:07:36.1366667+00:00

Hi @Ivan Svoboda

Please accept our sincere apologies for the delay in response. Our backend team has implemented a fix, and we kindly request you to retry creating the SMTP usernames to confirm if the issue is now resolved.

We will proceed with closing this request for now; however, please let us know if you are still facing any issues or require further assistance—we will be happy to help.

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  1. AI answer

    2026-04-14T10:37:36.5733333+00:00

    This behavior is caused by how Azure Communication Services manages SMTP usernames as ARM resources, not by anything left in the new ACS instance.

    SMTP usernames are first‑class resources of type Microsoft.Communication/communicationServices/smtpUsernames. Each username is globally constrained by the ARM resource name rules and is tracked independently of the parent instance. When an ACS resource group is deleted, the underlying platform can retain metadata for those smtpUsernames for some time, so attempts to recreate the same username strings can return 409 The username is not available even though:

    • GET on the username under the new ACS returns 404, and
    • listing smtpUsernames on the new ACS returns an empty collection.

    This is consistent with the documented model for the resource:

    {
  "type": "Microsoft.Communication/communicationServices/smtpUsernames",
  "apiVersion": "2026-03-18",
  "name": "string",
  "properties": {
    "entraApplicationId": "string",
    "tenantId": "string",
    "username": "string"
  }
}

    The name and properties.username are both required; the platform enforces uniqueness for username beyond the scope of a single ACS instance. Deleting the parent ACS or its resource group does not immediately free those usernames for reuse.

    Given the available information, there is no client‑side fix or additional ARM/Bicep change that will force release of those specific username strings. The only supported options are:

    1. Use different SMTP username strings (which is why new, previously unused usernames succeed), or
    2. Open a support request with Azure to have the stale smtpUsernames entries cleaned up or released so the original strings can be reused.

    Until the backend metadata is cleared by the service team, any attempt to recreate the same SMTP usernames will continue to fail with 409.

    References:

    AI-generated content may be incorrect. Read our transparency notes for more information.

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