Cannot send email (SMTP-Port 25)

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Cannot send email (SMTP-Port 25)

Nicola Ferrini 25 MVP

Cannot send email (SMTP-Port 25) in my subscription. How can I activate this?

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Answer 1

Jilakara Hemalatha 6,200 Microsoft External Staff Moderator

Hi Nicola Ferrini,

Port 25 is only eligible for Enterprise Agreement subscriptions. Check if your Azure subscription type is eligible for the port 25 unblock request.

It's possible to use port 25 for outbound communication on Azure App Service and Azure Functions through the virtual network integration feature or when App Service Environment v3 is used.

If unblocking port 25 is not feasible, consider configuring Postfix to use alternative ports that are commonly allowed for email transmission, such as port 587 (sending) or 465 (SMTPS).

For Enterprise Dev/Test subscriptions, outbound traffic on port 25 (SMTP) is blocked by default. However, it is possible to request the removal of this restriction. To initiate the request, navigate to the Diagnose and Solve Problems section within your Azure Virtual Network resource in the Azure portal. Locate and run the "Cannot send email (SMTP - Port 25)" diagnostic.

Please refer below screenshots:

User's image

Please check with your ISP has a policy of blocking outbound port 25 connections, except to their official outgoing SMTP servers and try with different network. also try

Navigate to control panel>>system and security>>Windows defender firewall>>Advanced settings>> Inbound rules>>add new rule>> select port>>Enter a custom rule name, e.g. TCP "Port 25">>allow the connection>>(Domain+Private+Public).

Check whether SMTP port 25 is opened or not. Open up a command prompt and run below command

 telnet <server_ip> 25

Please find below official documentation for reference:
https://v4.hkg1.meaqua.org/en-us/azure/virtual-network/troubleshoot-outbound-smtp-connectivity?wt.mc_id=knowledgesearch_inproduct_azure-cxp-community-insider#enterprise-and-mca-e

Hope this helps! Please let me know if you have any other queries.

Nicola Ferrini 25 Reputation points MVP

2025-09-04T09:25:53.74+00:00

Issue summary After moving an existing Azure Virtual Machine to a different subscription, outbound email stopped working. The Exchange server can receive mail, but it can no longer send mail. This started immediately after the subscription change. It appears that outbound SMTP on TCP port 25 is being blocked at the subscription level.

Recent changes

The VM was moved from one Azure subscription to another. No other configuration changes were made to the VM/Exchange server or to network security rules as part of this move.

What I’m requesting On the previous subscription I had an approved exception to allow SMTP traffic on port 25. After the move, outbound mail is failing, which suggests the port 25 block is in effect again on the new subscription.

Please re-enable/unblock outbound SMTP (TCP 25) for this new subscription (same as my prior exception), or advise on the correct process/form to request this exemption. Obvoiously I'll provide you the subscription ID

Logs / errors Outbound messages remain queued on the Exchange send connector and do not leave the server. If needed, I can provide specific event logs and connector/queue details.

Thank you!

Nicola
Nicola Ferrini 25 Reputation points MVP

2025-09-04T09:53:46.1+00:00

I recevied this message

The diagnostic did not detect any issues
Jilakara Hemalatha 6,200 Reputation points Microsoft External Staff Moderator

2025-09-04T11:56:17.68+00:00

Hi Nicola Ferrini,

Can you check if the port is opened from the server or not?

Test-NetConnection -ComputerName smtp.office365.com -Port 25
Jilakara Hemalatha 6,200 Reputation points Microsoft External Staff Moderator

2025-09-05T04:01:43.28+00:00

Hi Nicola Ferrini,

I wanted to follow up regarding the SMTP Port 25 issue. Could you please confirm if the problem has been resolved, or are you still experiencing difficulties sending emails over port 25?
Nicola Ferrini 25 Reputation points MVP

2025-09-05T06:40:41.2333333+00:00

Hello,

Unfortunately, I am still unable to use outbound TCP port 25 on my Azure VM.

Also I continue to receive errors in the Microsoft Exchange Queue
Jilakara Hemalatha 6,200 Reputation points Microsoft External Staff Moderator

2025-09-05T09:29:22.9066667+00:00

Hi Nicola Ferrini,

To investigate this issue further, could you kindly share the type of Azure subscription you are using?
Nicola Ferrini 25 Reputation points MVP

2025-09-05T11:04:00.17+00:00

Hello,

it's a benefit for Microsoft MVP. I have an annual amount of Azure credits.
Jilakara Hemalatha 6,200 Reputation points Microsoft External Staff Moderator

2025-09-05T18:50:54.45+00:00

Hi Nicola Ferrini,

Thank you for confirming.

For Enterprise Dev/Test subscriptions, port 25 is blocked by default. However, it is possible to request removal of this block.

For all other subscription types, the Azure platform blocks outbound SMTP connections on TCP port 25 for deployed VMs, and this block cannot be removed.

Recommendation:

It is strongly recommended to use an authenticated SMTP relay service instead of relying on port 25. Common options include connecting via TCP port 587 (sending) or 465 (SMTPS), which remain open and are appropriate for sending emails. Microsoft services such as Azure Communication Services can also be used as a reliable alternative for sending email from Azure resources.

Reference: https://v4.hkg1.meaqua.org/en-us/exchange/mail-flow-best-practices/how-to-set-up-a-multifunction-device-or-application-to-send-email-using-microsoft-365-or-office-365#requirements-for-microsoft-365-or-office-365-smtp-relay

Kindly let us know if the above helps or you need further assistance on this issue.
Nicola Ferrini 25 Reputation points MVP

2025-09-07T16:37:54.92+00:00

hi Jilakara Hemalatha,
may I use these ports for Exchange Hybrid deployment?

Many thanks

Nicola
Jilakara Hemalatha 6,200 Reputation points Microsoft External Staff Moderator

2025-09-08T06:09:04.1566667+00:00
Hi Nicola Ferrini,

For Exchange Hybrid deployment:

Port 25 is the only supported port for server-to-server mail flow between on-premises Exchange and Exchange Online. It must be open bidirectionally.

Port 587 is for client-to-server SMTP submission (e.g., Outlook, devices), not hybrid mail flow.

Port 465 is unsupported for Exchange.

Your hybrid connectors which handle mail flow between your on-premises Exchange servers and Exchange Online always use port 25.

You cannot configure hybrid connectors to use port 587 or 465. Attempting to do so will break hybrid mail flow and is unsupported by Microsoft

SMTP relay services (e.g., SendGrid or Microsoft 365 SMTP relay over ports 587 or 465) are suitable for sending mail from applications or devices, but they cannot be used for hybrid mail flow between Exchange servers.

Reference: https://v4.hkg1.meaqua.org/en-us/exchange/plan-and-deploy/deployment-ref/network-ports

https://v4.hkg1.meaqua.org/en-us/exchange/mail-flow/connectors/receive-connectors
Nicola Ferrini 25 Reputation points MVP

2025-09-08T06:19:43.6966667+00:00

OK. Thank you for your support.
Jilakara Hemalatha 6,200 Reputation points Microsoft External Staff Moderator

2025-09-09T02:24:51.2466667+00:00

Hi Nicola Ferrini,

If the answer is helpful, please click "Accept Answer" and kindly upvote. It will be helpful to community members as well.

Thanks.

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Cannot send email (SMTP-Port 25)

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