Cant BCC comments

Cant BCC comments

Whats the reason why we cant bcc to add to comments?
We are getting this error: 
We are unable to process this email.

WHY DID THIS HAPPEN?
We suspect the below reasons to be the cause of failure :

1. You might have mis-spelt the email address.
2. You might have added the email address in BCC


I used the copy option within the comments to copy the email address. It works fine if I cc, just not bcc.