What is a 'Mail block - spam detected' error?

The recipient’s email server has blocked your email because the content resembles spam email content. 
This mail block is often triggered because, 

• the recipient server is set up to detect spam emails 
• your reply-to address or brand name has a low reputation 

We treat this as a soft bounce because some mail servers and email providers respond with false or incorrect error codes.

To know more about Email Bounces and Error Codes, click here.