SBS2003 - SMTP Error 450 - Try again later (but only on 1 customer)

Hi.  I support multiple customers.  They have an employee that uses att.net for his e-mail at home.  E-Mails sent to him are delayed at least 5 hours, and sometimes never leave the queue.  The strange thing is that I do not have this problem sending to him from another customer sbs2003 machine.

here is the smtp log info:

2006-04-26 05:52:50 204.127.134.23 OutboundConnectionResponse SMTPSVC1 DELL01 - 25 - - 450+busy+-+please+try+later 0 0 27 0 31 SMTP - - - -
2006-04-26 05:52:50 204.127.134.23 OutboundConnectionCommand SMTPSVC1 DELL01 - 25 QUIT - - 0 0 4 0 46 SMTP - - - -
2006-04-26 05:52:50 12.102.240.23 OutboundConnectionResponse SMTPSVC1 DELL01 - 25 - - 450+busy+-+please+try+later 0 0 27 0 16 SMTP - - - -
2006-04-26 05:52:50 12.102.240.23 OutboundConnectionCommand SMTPSVC1 DELL01 - 25 QUIT - - 0 0 4 0 16 SMTP - - - -
2006-04-26 05:52:51 204.127.134.23 OutboundConnectionResponse SMTPSVC1 DELL01 - 25 - - 450+busy+-+please+try+later 0 0 27 0 15 SMTP - - - -
2006-04-26 05:52:51 204.127.134.23 OutboundConnectionCommand SMTPSVC1 DELL01 - 25 QUIT - - 0 0 4 0 31 SMTP - - - -
2006-04-26 05:52:51 12.102.240.23 OutboundConnectionResponse SMTPSVC1 DELL01 - 25 - - 220+worldnet.att.net+-+Maillennium+ESMTP/MULTIBOX+mtiwmxc14+#45 0 0 63 0 15 SMTP - - - -
2006-04-26 05:52:51 12.102.240.23 OutboundConnectionCommand SMTPSVC1 DELL01 - 25 EHLO - roilabels.com 0 0 4 0 15 SMTP - - - -
2006-04-26 05:52:51 12.102.240.23 OutboundConnectionResponse SMTPSVC1 DELL01 - 25 - - 250-worldnet.att.net 0 0 20 0 203 SMTP - - - -
2006-04-26 05:52:51 12.102.240.23 OutboundConnectionCommand SMTPSVC1 DELL01 - 25 QUIT - - 0 0 4 0 203 SMTP - - - -
2006-04-26 05:52:51 12.102.240.23 OutboundConnectionResponse SMTPSVC1 DELL01 - 25 - - 221+worldnet.att.net 0 0 20 0 218 SMTP - - - -

If I telnet gateway.att.net 25 using customer #1 sbs2003 machine, I get 450 - busy please try later

Same command from another customer, and they get the connection.

Emails from comcast and yahoo zip straight to the guy's e-mail address, and dnsstuff.com has a new e-mail tester that passes.

I am not using a smarthost on either machine, DNS straight through.

I have checked my DNS to verify all machines on network (including server) point to the SBS2003 server for DNS and that SBS2003 is forwarding to 2 DNS servers, which have the up to date info on the gateway2.att.net and gateway1.att.net

HELP!
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kurtbmillerAsked:
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SembeeCommented:
I think the only fix for this problem is going to be to use the ISPs server as a smart host with an additional SMTP Connector for just this domain.
AT&T are refusing to accept the connection, probably some kind of throttling, and there is nothing you can do about it. You could try asking AT&T for any reasons, but they will probably refuse to talk to you - most ISPs will not say a word about their antispam techniques.

Simon.
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Jeffrey Kane - TechSoEasyPrincipal ConsultantCommented:
We had similar problems with a client a couple weeks ago with this exact same issue.. the problems were primarily AT&T related and have since cleared up.  However, you want to be sure that you have a valid PTR record for the Reverse DNS entry and it couldn't hurt to also have an SPF record in your Zone file.  Recent adjustments in worldnet.att.net's protocols seem to have created stronger authentication requirements.

Jeff
TechSoEasy
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