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.