Azure Communication Services Email Failure: "Unrouteable sender address"

Question

Azure Communication Services Email Failure: "Unrouteable sender address"

Vikrant 20

Hello,

We are using multiple Azure Communication Services (ACS) instances across our Azure subscriptions for sending emails. However, we are observing inconsistent behavior:

Some ACS instances are working fine and sending emails successfully.
Others are failing when we try to send emails specifically to test email services like Yopmail.

Unrouteable sender address
MSG=
FQDN=smtp.yopmail.com
IP=87.98.164.155
LRT=8/7/2025 11:05:22 AM

User's image

Additional Details:

We were using a test sender address from yopmail.com while troubleshooting.

In some ACS resources, the email sends fine. In others, it fails with the above error.

We have verified that DeliveryStatus shows “Failed” for the affected ones.

No major configuration difference is visible in the Azure Portal between the working and non-working ACS instances.

Questions:

Why is this "Unrouteable sender address" error occurring in some ACS resources but not others?

Does ACS have restrictions or different internal rules across regions or instances?

Is yopmail.com (a disposable domain) being blocked selectively or entirely by ACS?

What are the best practices for configuring sender domains and ensuring consistent delivery across ACS instances?

How should we test email functionality in ACS if we don’t yet have a verified custom domain?

Any help in understanding this discrepancy would be appreciated.

Vimal Lalani 170 Reputation points Microsoft External Staff Moderator

2025-08-07T18:12:05.62+00:00

Hi Vikrant

Thank you for posting your question on the Microsoft Q&A forum.

As you mentioned that you are observing inconsistent behavior across multiple Azure Communication Services (ACS) instances while sending emails—specifically, some instances successfully send to addresses like Yopmail, while others fail with a “Unrouteable sender address” error, despite no apparent configuration differences. You are also trying to understand if this issue relates to regional restrictions, domain blocking, or sender domain configuration.

After reviewing the error and available details, it appears the actual delivery status indicates MailboxUnavailable. This status generally means that the recipient's mailbox is full or temporarily unavailable, which results in a hard bounce. When repeated hard bounces occur to the same address, ACS may add that specific email ID to its global suppression list.

Note: It is important to note that the global suppression list does not block the entire domain, but only the individual failing recipient email addresses.

Reference: Understand sender reputation in Azure Communication Services email - An Azure Communication Services article | Microsoft Learn

If you do not yet have a custom verified domain, you can still test email functionality using Azure-managed domains, which are supported by the Email Communication Service.

Reference: Add Azure Managed Domains to Email Communication Service - An Azure Communication Services quick start guide | Microsoft Learn

For production use or consistent delivery, it is recommended to follow best practices for configuring custom verified domains.

Reference: Add custom verified email domains - An Azure Communication Services article | Microsoft Learn

Feel free to post back if you have any follow-up questions or need further assistance.
Vikrant 20 Reputation points

2025-08-08T06:27:24.19+00:00

Hello Vimal Lalani

Thank you for your input on the issue.

However, my concern is that I am able to send emails to the same email address using a different Azure Communication Service (ACS) instance. The issue only occurs with a newly created ACS instance.

I suspect there might have been recent updates to ACS policies that restrict sending emails to disposable email domains like Yopmail.

This issue is consistently occurring across all newly created ACS instances, whereas the ones created earlier are still functioning as expected.
Vimal Lalani 170 Reputation points Microsoft External Staff Moderator

2025-08-11T11:15:17.8366667+00:00

Hi @Vikrant
I am checking on your concern with our internal team and will update you once I have more information.

Thanks

Vimal

Your answer

Vimal Lalani 170 Reputation points Microsoft External Staff Moderator

2025-08-07T18:12:05.62+00:00

Hi Vikrant

Thank you for posting your question on the Microsoft Q&A forum.

As you mentioned that you are observing inconsistent behavior across multiple Azure Communication Services (ACS) instances while sending emails—specifically, some instances successfully send to addresses like Yopmail, while others fail with a “Unrouteable sender address” error, despite no apparent configuration differences. You are also trying to understand if this issue relates to regional restrictions, domain blocking, or sender domain configuration.

After reviewing the error and available details, it appears the actual delivery status indicates MailboxUnavailable. This status generally means that the recipient's mailbox is full or temporarily unavailable, which results in a hard bounce. When repeated hard bounces occur to the same address, ACS may add that specific email ID to its global suppression list.

Note: It is important to note that the global suppression list does not block the entire domain, but only the individual failing recipient email addresses.

Reference: Understand sender reputation in Azure Communication Services email - An Azure Communication Services article | Microsoft Learn

If you do not yet have a custom verified domain, you can still test email functionality using Azure-managed domains, which are supported by the Email Communication Service.

Reference: Add Azure Managed Domains to Email Communication Service - An Azure Communication Services quick start guide | Microsoft Learn

For production use or consistent delivery, it is recommended to follow best practices for configuring custom verified domains.

Reference: Add custom verified email domains - An Azure Communication Services article | Microsoft Learn

Feel free to post back if you have any follow-up questions or need further assistance.
Vikrant 20 Reputation points

2025-08-08T06:27:24.19+00:00

Hello Vimal Lalani

Thank you for your input on the issue.

However, my concern is that I am able to send emails to the same email address using a different Azure Communication Service (ACS) instance. The issue only occurs with a newly created ACS instance.

I suspect there might have been recent updates to ACS policies that restrict sending emails to disposable email domains like Yopmail.

This issue is consistently occurring across all newly created ACS instances, whereas the ones created earlier are still functioning as expected.
Vimal Lalani 170 Reputation points Microsoft External Staff Moderator

2025-08-11T11:15:17.8366667+00:00

Hi @Vikrant
I am checking on your concern with our internal team and will update you once I have more information.

Thanks

Vimal

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Azure Communication Services Email Failure: "Unrouteable sender address"

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