Exchange 2013: 451 4.7.0 Temporary server error. Please try again later. PRX2

In my fish Exchange 2013 test environment I could not send any mails, all Exchange 2013 reported the error "451 4.7.0 Temporary server error. Please try again later PRX2":

220 EX1.frankysweb.local Microsoft ESMTP MAIL Service ready at Sat, 31 Aug 2013 19:30:39 +0200

HELO EX1
250 EX1.frankysweb.local Hello [192.168.200.101]

MAIL From:
250 2.1.0 Transmitter OK

RCPT To:
250 2.1.5 Recipient OK

DATA
354 Start mail input; end with .
From: "test@frankysweb.de"
To: "frank@frankysweb.de"
Subject: test

test

.
451 4.7.0 Temporary server error. Please try again later. PRX2

The cause is simple, I had forgotten to create a reverse lookup zone on my DNS server.

After I had created a reverse lookup zone and created the corresponding PTR entries for the Exchange servers, sending mail also worked

220 EX1.frankysweb.local Microsoft ESMTP MAIL Service ready at Sat, 31 Aug 2013 19:45:45 +0200

HELO EX1
250 EX1.frankysweb.local Hello [192.168.200.101]

MAIL From:
250 2.1.0 Transmitter OK

RCPT To:
250 2.1.5 Recipient OK

DATA
354 Start mail input; end with .
From: "test@frankysweb.de"
To: "frank@frankysweb.de"
Subject: test

test

.
250 2.6.0 [InternalId=360777252926] Queued mail for delivery

So if you ever encounter this error, remember the Reverse Zone and the PTR Records.