The Problem:
You are trying to send an email in C# from your website (using Exchange as the email server), but you get the error message
Mailbox unavailable. The server response was: 5.7.1 Message rejected as spam by Content Filtering.
whenever you send the email. It is clearly not spam.
The Cause:
You are triggering a spam filter based on the number of words and characthers in the sample email you’re sending.
The Solution:
You should increase the number of words in the body of your email by five or more. It’s strange, but it works.