SmtpStatusCode 550 when sending emails to Trans IP emails

Question

SmtpStatusCode 550 when sending emails to Trans IP emails

Lisa 60

Hello,

I am using Azure communication services to send emails. This is working fine for the most part other than when I try to send emails to Trans IP emails. When this occurs I get the following response:
DeliveryStatus: Failed

SmtpStatusCode: 550

EnhancedSmtpStatusCode: 5.1.0

RecipientMailServerHostName: mx.transip.email

IsHardBounce: False

FailureReason: MailboxUnavailable

We are able to send emails to all other providers.

We have contacted trans IP and they said that these emails are not arriving at their server and that it is possible that they are on an internal block list. We are able to send emails from other email accounts to the Trans IP accounts that are failing.

Is it correct that they are possibly on an internally blocked list from Microsoft? And if so is there a way to make an exception for our emails.

Many thanks,

Lisa

Aditya N 2,990 Reputation points Microsoft External Staff Moderator

2026-06-11T12:22:56.7166667+00:00
Hello @Lisa

Thank you for reaching out Microsoft Q&A. SmtpStatusCode: 550 - this error indicates that the server has rejected the email. Please could you

Check ACS email delivery logs

In Azure portal → your Communication Services resource → enable Diagnostic settings

Use Log Analytics / ACS email logs to find the delivery events for the affected recipient(s)

Review sender reputation and suppression list behavior

ACS provides mechanisms intended to protect sender reputation and includes guidance around managing suppression/reputation to improve deliverability.

If you’re sending at any notable volume, reputation effects can show up selectively.
Lisa 60 Reputation points

2026-06-12T09:01:21.04+00:00

Hello @Aditya N ,

Thank you for your reply. I have the email delivery logs turned on and the information from my first message relates to the EmailStatusUpdateOperational logs I am seeing with a failed delivery status. Is there any additional information I can get from those logs?

With your comments on the sender reputation, do you mean that Microsoft is blocking the emails that I am trying to send or that the sender reputation of our emails is poor which is causing this issue?
Lisa 60 Reputation points

2026-06-16T09:16:10.0833333+00:00

Hello @Aditya N ,

Is there any further information I can provide?
Many thanks,

Lisa
Aditya N 2,990 Reputation points Microsoft External Staff Moderator

2026-06-17T13:54:48.2533333+00:00

Hello @Lisa

Please could you let us know whether the response provided was helpful.

2 answers

Your answer

Aditya N 2,990 Reputation points Microsoft External Staff Moderator

2026-06-11T12:22:56.7166667+00:00

Hello @Lisa

Thank you for reaching out Microsoft Q&A. SmtpStatusCode: 550 - this error indicates that the server has rejected the email. Please could you

Check ACS email delivery logs

In Azure portal → your Communication Services resource → enable Diagnostic settings

Use Log Analytics / ACS email logs to find the delivery events for the affected recipient(s)

Review sender reputation and suppression list behavior

ACS provides mechanisms intended to protect sender reputation and includes guidance around managing suppression/reputation to improve deliverability.

If you’re sending at any notable volume, reputation effects can show up selectively.
Lisa 60 Reputation points

2026-06-12T09:01:21.04+00:00

Hello @Aditya N ,

Thank you for your reply. I have the email delivery logs turned on and the information from my first message relates to the EmailStatusUpdateOperational logs I am seeing with a failed delivery status. Is there any additional information I can get from those logs?

With your comments on the sender reputation, do you mean that Microsoft is blocking the emails that I am trying to send or that the sender reputation of our emails is poor which is causing this issue?
Lisa 60 Reputation points

2026-06-16T09:16:10.0833333+00:00

Hello @Aditya N ,

Is there any further information I can provide?
Many thanks,

Lisa
Aditya N 2,990 Reputation points Microsoft External Staff Moderator

2026-06-17T13:54:48.2533333+00:00

Hello @Lisa

Please could you let us know whether the response provided was helpful.

Answer 1

Aditya N 2,990 Microsoft External Staff Moderator

Hello @Lisa

Azure ACS tries to manage sender reputation, in your scenario it might be due to sender reputation. Providing the official documentation below for the same.

User's image

https://learn.microsoft.com/en-us/azure/communication-services/concepts/email/sender-reputation-managed-suppression-list

We suggest using custom domain for email.

0 comments

Answer 2

For Azure Communication Services Email, SmtpStatusCode: 550 with EnhancedSmtpStatusCode: 5.1.0 and FailureReason: MailboxUnavailable indicates the recipient domain’s infrastructure is rejecting or not accepting the message for that mailbox, not that Azure Communication Services itself is blocking the traffic.

From the Azure Communication Services email documentation, 550/Mailbox Unavailable means the recipient mailbox or its environment is unavailable or not accepting mail:

550 | Mailbox Unavailable | The recipient's mailbox is unavailable to receive emails. The mailbox might be full or might have a temporary problem.

Given that:

Messages to other providers succeed.
TransIP reports the messages are not reaching their servers.

The most likely causes are:

A block or suppression at the sending side (Azure Communication Services email) for that destination domain or specific addresses.
A reputation/suppression issue due to previous bounces or complaints, causing ACS to treat those recipients as suppressed and not attempt delivery.

In Azure Communication Services email, such conditions are handled through managed suppression and sender reputation controls. There is no tenant-level “allow list” that can be configured directly to override a managed suppression list; exceptions are not exposed as a self-service feature. When ACS marks a recipient as suppressed (for example, after repeated bounces with 5xx codes), further sends to that address or domain can be blocked before reaching the recipient’s MX.

Recommended actions:

Review the detailed delivery status and bounce information in the ACS email telemetry for those TransIP recipients to confirm repeated 550/MailboxUnavailable or similar patterns.
Verify the recipient addresses at TransIP are valid, active, and not over quota.
If the addresses are valid and the issue persists only from ACS, open a support case with Azure Support for Azure Communication Services email, providing:
- The affected recipient addresses and domain.
- Message IDs and timestamps.
- The full SMTP status details (550 5.1.0 MailboxUnavailable, mx.transip.email).

Support can check whether those recipients or that domain are on an internal suppression/block list and, if appropriate, remove them. Direct self-service exception management for internal ACS suppression lists is not available.

References:

Understand sender reputation in Azure Communication Services email

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SmtpStatusCode 550 when sending emails to Trans IP emails

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