This is a weird scenario.

I have two websites hosted on a Shared Multi-Domain Windows hosting server.

I have setup e-mail on both websites and using an aspx script to send e-mails using the SMTP service provided by the Host.

For Website A, its working just fine, for Website B it gives me an error: The SMTP server requires a secure connection or the client was not authenticated. The server response was: SMTP authentication is required.

SMTP Port is 25, SSL is False

I have checked both the scripts, they are exactly same. What's weird is that when I run the aspx script of Website B on localhost (IIS Express within Visual Studio), the mails are successfully sent!

The only difference in the hosting of two websites is that Website B is using SSL Certificate from Let's Encrypt. I suspect this is the reason of above error, however, I am not sure how that affects traffic on SMTP server.

Also, just to provide additional info - I have disabled ModSecurity on website using Web.Config file.

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