SMTP-Error 451 while using smarthost for mail delivery

[b1 <forum@b1online.de>]

Hello alltogether

Currently I am trying to set up a Debian Lenny Server, but I am stuck at
mail delivery. The server I am trying to set up, has no FQDN, so I used
my ISP-Mailserver as a smarthost (I enabled the proper SMTP
authentication in exim4).
This setup worked a few days, before I suddenly received 451 errors.
At first I thought it to be a problem with Exim4, so I switched to
postfix. But postfix also gives me the 451 error:

Apr 22 15:53:31 openshoolproxy postfix/smtp[24368]: 413CF26EF9:
to=<webservice@b1online.de>, orig_to=<root@openshoolproxy>,
relay=smtp.strato.com[81.169.145.132]:25, delay=2137,
delays=2136/0.05/0.33/0.12, dsn=4.0.0, status=deferred (host
smtp.strato.com[81.169.145.132] said: 451 Local Error (in reply to end
of DATA command))

To further troubleshoot my problem I used telnet to connect to
smtp.strato.com and to send a mail manually.

This is what I did:

telnet smtp.strato.de 25
Trying 81.169.145.133...
Connected to smtp.strato.de.
Escape character is '^]'.
220 smtp.passthru
EHLO openshoolproxy
250-smtp.passthru
250-ENHANCEDSTATUSCODES
250-8BITMIME
250-DELIVERBY
250-SIZE 104857600
250-AUTH PLAIN LOGIN CRAM-MD5
250 HELP
AUTH LOGIN
334 VXNlcm5hbWU6
d2Vic2VydmljZUBiMW9ubGluZS5kZQ==
334 UGFzc3dvcmQ6
xxxxMYPASSWORD_IN_BASE64xxxx
235 2.0.0 OK Authenticated
mail from: webservice@b1online.de
250 2.1.0 <webservice@b1online.de> Sender ok
rcpt to: benedikt@b1online.de
250 2.1.5 <benedikt@b1online.de> Recipient ok
DATA
354 Start mail input; end with <CRLF>.<CRLF>
test
.
451 Local Error




Very odd. After this fail I tried the same procedure from my arch linux
box at home. It succeeded:




telnet smtp.strato.com 25
Trying 81.169.145.132...
Connected to smtp.strato.com.
Escape character is '^]'.
220 post.strato.de [fruni mo3] ESMTP RZmta 23.0 ready; Thu, 22 Apr 2010
15:55:55 +0200 (MEST)
EHLO openshoolproxy
250-post.strato.de [fruni mo3] greets 79.230.91.82
250-ENHANCEDSTATUSCODES
250-8BITMIME
250-PIPELINING
250-DELIVERBY
250-SIZE 104857600
250-AUTH PLAIN LOGIN CRAM-MD5
250-STARTTLS
250 HELP
AUTH LOGIN
334 VXNlcm5hbWU6
d2Vic2VydmljZUBiMW9ubGluZS5kZQ==
334 UGFzc3dvcmQ6
xxxxMYPASSWORD_IN_BASE64xxxx
235 2.0.0 OK Authenticated
mail from: webservice@b1online.de
250 2.1.0 <webservice@b1online.de> Sender ok
rcpt to: benedikt@b1online.de
250 2.1.5 <benedikt@b1online.de> Recipient ok
DATA
354 Enter data for mail with id g0718am3MD7AJw
test mich
.
250 queued as g0718am3MD7AJw


This looks very strange to me. Independently from my mailserver the
telnet command should have succeeded on both machines. But it failed on
my debian box. What could be wrong? Is this a problem of my setup, or is
my ISP blocking something?

Thanks in advance

Benedikt