Exchange 2010: Why does the recipient accept out-of-office emails when they omit the Postfix smart host?

With the internal recipients everything works very well. Exchange 2010 Out-of-office emails are sent and received. For external recipients, out-of-office emails are sent well, but are not accepted by external recipients:

he said: 550 Action requested not taken: mailbox not available (in response to
MAIL FROM command))

When I omit the post-office smart host, the emails are received by external recipients, but when I go through the smart host after the correction does not work.

Here is a diagram: Mails outside the office are rejected:

  • Incoming mail: Internet -> Postfix -> Exchange 2010
  • Outgoing mail: Internet <- Postfix <- Exchange 2010

Emails are accepted outside the office:

  • Incoming mail: Internet -> Postfix -> Exchange 2010
  • Outgoing mail: Internet <- Exchange 2010

Do you know why Postfix interferes and prevents mail from being rejected outside the office (550 mailboxes unavailable)?

Do you know if it is dangerous to leave directly without a smart host?

Thanks in advance