SMTP outbound mail problem on port 587

Question

I have a web app running as an App Service in Azure on Premium P2V3 instance.
SMTP mail is failing on port 587 with SSL enabled.

Mail was working until I upgraded the instance.

Mail from my other servers is working to this mail server with the same settings and identity.

The Mail service provider assures me that they are not blocking the list of outbound IP addresses I sent them and they are not throttling us and they are not seeing any failed connections.
The web app is a running on ASP.NET framework 4.8

Any suggestions on how I can track this problem down.

Thanks

Answer 1

Hi @Darryl Buckby
It seems that you're encountering SMTP delivery problems on port 587 after upgrading your Azure App Service instance.

1.Make sure your app's SMTP client is set to use StartTLS (EnableSsl = true), which is the correct method for port 587.
2.Check your web app logs or Application Insights for any SMTP exceptions or timeout errors. This will help pinpoint where the failure occurs.
3.Ensure your App Service instance can connect to the SMTP host on port 587.
4.After upgrading, your App Service may now use different outbound IPs. Make sure these IPs are allowed by your SMTP provider.
5.Use a tool like Telnet to test the connection to your SMTP server. You can do this by running telnet your.smtp.server 587 from a command line on a local machine or an Azure VM in the same virtual network to see if it connects.
https://learn.microsoft.com/en-us/azure/virtual-network/troubleshoot-outbound-smtp-connectivity
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