Exchange 2003 Hopping mail between ISP and server

Hi,
I seem to have mail bouncing from our exchange 2003 server back to the ISP indefinitly, I found out why this was happening : The e-mails that are bouncing contain invalid adress's in the CC field or the To field and vice versa. The system is accepting the valid one's and sending the invalid one's back to the ISP. Then exchange connects again, downloads the mail and the process starts all over again. Filtering is switched on for recipients and SMTP as well as a postmaster box to send undeliverable mail. But the symptoms continue....
Opie12Asked:
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Opie12Connect With a Mentor Author Commented:
Point well made.
Unfortunatly with budgets being tight as it is, implementing new solutions with costs may not be possible. As for the problem I going to keep hacking away at it.
Thank you very much for the help with ideas so far
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tmeunierConnect With a Mentor Commented:
Well, we have to narrow down the terminology a little bit, here, because Exchange doesn't "connect" and "download" email from an ISP, typically.  Unless you're talking about something like the POP3 connector that comes with Small Business Server, or a third-party product that provides that sort of functionality.  Is that the case, or is this a standard Exchange Server installation, where the sending email server initiates an SMTP session with your Exchange server on port 25, and sends the mail?

So, assuming that it's the latter, standard case.... You're saying that if John.Doe@elsewhere.com sends an email to Al@yourdomain.com, and CC:s Bob@yourdomain.com, Charlie@yourdomain.com, and Debby@yourdomain.com, and Charlie no longer exists, that a nondelivery reciept will be generated.   And then you say that the email server at ELSEWHERE.COM later initiates another attempt to send mail, re-sending messages for Al, Bob, Charlie, and Debby, even though the valid users have already received the mail?  

If this is the case, then I would say that elsewhere.com's email server is broken, or that there is a messaging hygiene product or other intermediary host, between the two systems with a broken SMTP implementation.
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Opie12Author Commented:
Hi, Well yes the first one, the exchange server uses a POP3 connector to fetch mail from the ISP. According to the ISP the exchange server is accepting all mail to that domain regardless if the user excists or not. Which shifts the problem back to me...

Thank you for your response!
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tmeunierConnect With a Mentor Commented:
Is there a way you can stop using the POP3 connector?  These implementations almost always have these problems.  Best solution is to get a fixed IP address from your ISP, update your Exchange server's A record and your organization's MX record in DNS to point to the  and open port 25 through your firewall for inbound SMTP, and let the Exchange Server handle the mail.  I'd say this is more a problem with your POP3 connector than with Exchange.
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