Authentication failed because the remote party has closed the transport stream - smtp error sending

Question

One of our home grown applications uses o365 to send emails via smtp. For some reason it just stopped working and these are the errors we are getting.

"SmtpEx: System.Net.Mail.SmtpException: Failure sending mail. ---> System.IO.IOException: Authentication failed because the remote party has closed the transport stream."

TLS 1.1 and 1.2 are setup as options on the server sending the message and we are using the following SMTP server

Texttenantname-com.mail.protection.outlook.com Port 25

I test on the server via powershell using the same credetials and smtp server it and works fine. I'm trying to get a hold of microsoft but been having a hard time.

Server 2012r2

user_5798900 · Answer

Just ran into the same problem, was able to get past it by specifying the security protocol (my case is a .NET 4 application, hence the casting):

ServicePointManager.SecurityProtocol = (SecurityProtocolType)48 | (SecurityProtocolType)192 | (SecurityProtocolType)768 | (SecurityProtocolType)3072;

Source: https://codeshare.co.uk/blog/how-to-fix-the-error-authentication-failed-because-the-remote-party-has-closed-the-transport-stream/

Authentication failed because the remote party has closed the transport stream - smtp error sending

1 Answers