I've got 2 IIS servers: "Local" IIS" that runs on my network and "Remote IIS" located on other network. Both IIS servers have exactly the same application installed used to send e-mails. An email used to send messages is from Exchange Online, Microsoft 365. Following the guide Configure Send Submission: I have configured "SMTP submission" for this email address, however, it only works from "Local IIS" and not from "Remote IIS", not sure why.

Remote IIS: Windows 2012R2, IIS 8

Local IIS, windows 10 Pro; IIS 10

There is nothing that blocks outgoing port 587 on "Remote IIS". For testing purposes my colleague created a simple app, that runs independently from IIS , that also sends email and it works fine using the same Exchange Online user. The problem seems to be on IIS server on that server. Any ideas why "Remote IIS" does't send email?

The error is:

ERROR: Code: GeneralFailure Trace: en System.Net.Mail.SmtpClient.Send(MailMessage message)   en Forms_Login.btopepe_Click(Object sender, EventArgs e) Message: Error al enviar correo.
  • Have you checked event viewer logs for any errors that may indicate an issue. How about IIS logs, have you checked those. Furthermore, for extra thoroughness, what happens if you disable the Windows Firewall temporarily and then try to send the email out with the IIS method? You might need to add the IIS app.exe file that sends the emails from the IIS method to the Windows Firewall even though the port 587 is allowed so those are the typical basic things I'd start with if it was me to ensure something simple is not being overlooked. Using another app can be deceiving sometimes. – Pimp Juice IT Jan 18 at 17:22
  • @PimpJuiceIT Thanks, I've already check with firewall disabled and this doesn't help. There's nothing in logs, I checked them too. Local site is on IIS version 10, and the one that doesn't work is on version 8, although not sure if this is relevant. – Okrx Jan 18 at 20:42

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