Message Blocked | 550 Envelope blocked - User Entry

[Kirubakaran]

Hello All,

Our GTM emails are getting blocked by few domains that are using mimecast's email security it seems. Following is the bounce back email response from those receivers, 

Message blocked
Your message to (username@domain.com) has been blocked. See technical details below for more information.

The response from the remote server was:
550 Envelope blocked - User Entry - mimecast community [Z23cWiIxOGidUNl_6-iWng.us321]

Any help?