Wondering about ": End-of-data rejected: "

Hello,

I'm experimenting with policy servers.
(this is with postfix 2.11.0)

Let's say that such a server, when invoked from a smtpd_end_of_data_restrictions, writes on STDOUT:

action=REJECT Mailbox is unavailable

This is then what I get in the SMTP conversation:

554 5.7.1 <END-OF-MESSAGE>: End-of-data rejected: Mailbox is unavailable

Where does this substring "<END-OF-MESSAGE>: End-of-data rejected: " come from?
Can it be avoided?

I must be missing something, but I've been unable to find the relevant info in the documentation...

Thanks in advance,
Axel