For the pop3 traffic to go through your sme server you need the following:
Sme server is not the final destination for the email (means it has assigned a different domain then the one you send mail to). If it is assigned the domain you are sending mail to, then it considers it is for itself, not asking the root dns who's the domain machine as long as it thinks it is itself and then it handles mail as usual trying to direct it to the user it is for and failing that notifying the admin.
Second, if you do not use the email system from the SME it would be a good ideea to disable it. My servers work flawlessly no matter if the mail is directed to them or not. I even have a pptp connection behind a firewall, still I can send and receive pop3 traffic from the outside world, as well as from the mail server within the sme server.
Third, your server is not behind a firewall when it has a public IP assigned to it(the vast majority of cases). As long as you are directing tests and you still have your original windows server in place I assume you do not have 2 different connections to the internet, so, maybe, check again the firewall thing. I can have a sme server behind a firewall, in most setups whould work fine to receive and send mails, only problem cames in when you are trying to receive mails from outside your internal network, when the firewall you have in place does not forward the necessary ports to your sme server.
If the sme gives you that error, then why don't you post the whole message, from what it sais there, it just sais the mail did not succeed. I imagine you set your sme as you default mail server (SMTP+POP3) and you are asking from mails on an account on another server.
Try this: In outlook express, set your EXTERNAL account (not the one on the sme server) and at smtp and POP3 settings set the machine name (ex. mail@somehost.com), then when you add an account created on your machine (sme server that is) use IP (ex. 10.0.0.1) as I imagine it is on an internal network. This will unconfuse you about which machine is on which domain, and will permit further troubleshooting. And please, give the whole error, not just the part where it sais there is an error.
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