Error while sending message! Please contact Zoho mail support

Error while sending message! Please contact Zoho mail support

Hi 

We keep getting the message when sending mail: 

Error while sending message! 

The Case 1 :   when i am reply email use "REPLY" . this action false . ERROR

The Case 2 :   when i am reply email use "COMPOSE" write new email . this action TRUE . successfull
 
Please contact Zoho mail support 

can you advise when this will be rectified please