External senders randomly and temporary receive SMTPSEND.DNS.NonExistentDomain NDR messages

On different moments of the day, the Exchange 2013 server bounces mail by sending an NDR message back to an external sender with SMTPSEND.DNS.NonExistentDomain NDR messages.

Mail was flowing okay with Exchange 2010.

Removed (uninstalled) all exchange 2010 servers and now only have one Exchange 2013 server running, to simplify things.

Two Domain Controllers running for internal .local domain, with integrated DNS. Exchange is accepting mail for external domain .eu. With Exchange 2010, mail domain was only configured in DNS of ISP. Created a split dns for Exchange 2013, but still to no avail.

How can I troubleshoot these mysterious SMTPSEND.DNS.NonExistentDomain NDR messages.

This log excerpt also shows that internal(?) mailflow suffers from this issue?
0,127.0.0.1:25,127.0.0.1:65452,+,,
1,127.0.0.1:25,127.0.0.1:65452,*,SMTPSubmit SMTPAcceptAnySender SMTPAcceptAuthoritativeDomainSender AcceptRoutingHeaders,Set Session Permissions
2,127.0.0.1:25,127.0.0.1:65452,>,"220 VOS03.vos.local Microsoft ESMTP MAIL Service ready at Sun, 14 Apr 2013 02:37:25 +0200",
3,127.0.0.1:25,127.0.0.1:65452,<,EHLO InboundProxyProbe,
4,127.0.0.1:25,127.0.0.1:65452,>,250-VOS03.vos.local Hello [127.0.0.1],
5,127.0.0.1:25,127.0.0.1:65452,>,250-SIZE 104857600,
6,127.0.0.1:25,127.0.0.1:65452,>,250-PIPELINING,
7,127.0.0.1:25,127.0.0.1:65452,>,250-DSN,
8,127.0.0.1:25,127.0.0.1:65452,>,250-ENHANCEDSTATUSCODES,
9,127.0.0.1:25,127.0.0.1:65452,>,250-STARTTLS,
10,127.0.0.1:25,127.0.0.1:65452,>,250-X-ANONYMOUSTLS,
11,127.0.0.1:25,127.0.0.1:65452,>,250-AUTH NTLM,
12,127.0.0.1:25,127.0.0.1:65452,>,250-X-EXPS GSSAPI NTLM,
13,127.0.0.1:25,127.0.0.1:65452,>,250-8BITMIME,
14,127.0.0.1:25,127.0.0.1:65452,>,250-BINARYMIME,
15,127.0.0.1:25,127.0.0.1:65452,>,250-CHUNKING,
16,127.0.0.1:25,127.0.0.1:65452,>,250 XRDST,
17,127.0.0.1:25,127.0.0.1:65452,<,MAIL FROM: inboundproxy@inboundproxy.com,
18,127.0.0.1:25,127.0.0.1:65452,*,08D0051A94EF7ECB;2013-04-14T00:37:25.784Z;1,receiving message
19,127.0.0.1:25,127.0.0.1:65452,>,250 2.1.0 Sender OK,
20,127.0.0.1:25,127.0.0.1:65452,<,RCPT TO: HealthMailboxc6e7762226694d7baf7d7da68cf5c043@vos.local,
21,127.0.0.1:25,127.0.0.1:65452,>,250 2.1.5 Recipient OK,
22,127.0.0.1:25,127.0.0.1:65452,<,DATA,
23,127.0.0.1:25,127.0.0.1:65452,>,354 Start mail input; end with <CRLF>.<CRLF>,
24,127.0.0.1:25,127.0.0.1:65452,*,,Proxy destination(s) obtained from OnProxyInboundMessage event
25,127.0.0.1:25,127.0.0.1:65452,*,,Message or connection acked with status Fail and response 554 5.4.4 SMTPSEND.DNS.NonExistentDomain; nonexistent domain
26,127.0.0.1:25,127.0.0.1:65452,>,451 4.7.0 Temporary server error. Please try again later. PRX2 ,
27,127.0.0.1:25,127.0.0.1:65452,<,QUIT,
28,127.0.0.1:25,127.0.0.1:65452,>,221 2.0.0 Service closing transmission channel,
29,127.0.0.1:25,127.0.0.1:65452,-,,Local

Sample of times on date 2013-04-14 that Exchange Server\V15\TransportRoles\Logs\FrontEnd\ProtocolLog\SmtpReceive\RECV20130414-1.LOG contains SMTPSEND.DNS.NonExistentDomain: