Email bounced from Zoho: 554 5.4.0 <mx2.zoho.com #4.4.1>

Email bounced from Zoho: 554 5.4.0 <mx2.zoho.com #4.4.1>

My friend continue having issue sending email to us, and gave me the bounced email with the following reason:

Remote Server returned '554 5.4.0 <mx2.zoho.com #4.4.1>'
..

1. What exactly the cause of ' 554 5.4.0 < mx2.zoho.com   #4.4.1>' ?
2. Is it related to the headers size limit? If yes, what are the limit by Zoho?

Thanks