554 5.2.3 MailPolicy violation Error delivering to mailboxes

I can't receive a email from my friend.
The email has 2 small attached files (PDF).

What's the problem?

----- The following addresses had permanent fatal errors -----
<taro@tran******.jp>
(reason: 554 5.2.3 MailPolicy violation Error delivering to mailboxes)

----- Transcript of session follows -----
... while talking to mx.zohomail.com.:
>>> DATA
<<< 554 5.2.3 MailPolicy violation Error delivering to mailboxes
554 5.0.0 Service unavailable