I'm sure I'm missing something simple, but I've reread similar posts several times and can't find it.  We've made these changes:

    DNS Server
        Changed MX record to point to mail2 and new Internet IP

    Mail / Spam Filter Server
        Added new IP to a NIC on the mail server .
        Told Spam Filter to listen to new IP, port 25 and forward to 127.0.0.1, port 28
        Changed iMail to listen to port 28 (not 25)

This arrangement filtered incoming mail, but rejected all outgoing mail. We can't ask clients to change Outlook to use new port.  They are all configured to use same incoming and outgoing mail server mail.isp.com

What am I missing?  I would really appreciate any help.

Dee,

There are several threads on this subject. The key point is that SpamFilter is designed to process you incoming email only. Your outgoing email must still go thru your existing SMTP server.

If you already added a new IP to the NIC, there is no need to change the port iMail listens on. You can then configure SpamFilter to forward email to eMail on the other IP and port 25. This allows all your clients to not change their configs, and still use iMail as their outgoing smtp server. 

If iMail takes control aof all IPs bound to the NIC, then the above solution will not work, and you will need to change ports. However, if your server(s) are behind a firewall, you can then configure SpamFilter to listen on a different port, 28 for ex., and then configure the firewall to take incoming traffic on port 25 and redirect it to SpamFilter's IP on port 28.

Following is a http://www.logsat.com/spamfilter/forums/showmessage.asp?messageID=474" CLASS="ASPForums" TITLE="WARNING: URL created by poster. - posting that deals with a configuration similar to yours:

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We assumed that you only had access to one single IP from your provider, and the suggestions we ghave before were based on that. Yes, of course if you have multiple internet IPs available (not internal LAN IP, they have to be internet IPs) things are much simpler.

It's extremely easy to bind multiple IPs to a single NIC card in one computer. We've prepared some screenshots on how to do so, http://www.logsat.com/spamfilter/images/ipbinding.jpg" CLASS="ASPForums" TITLE="WARNING: URL created by poster. - click here to view them.

We see that your MX record for comcast.net points to mx00.comcast.net (24.153.64.1). If that is what your clients use as their outgoing smtp server, and you do not want to change your clients configs (which is what we recommend), you will need to have SpamFilter listen on a different IP, let's say 24.153.64.2 for example. The only thing you will have to change then is the MX record so that it points to SpamFilter's IP, 24.153.64.2.

By doing that incoming email will go where the MX record direct it, SpamFilter in this case. SpamFilter will then forward it to your IMail server on 24.153.64.1. Your clients will not change a thing, and still point to your IMail at 24.153.64.1.

Hope this helps.

Roberto Franceschetti
LogSat Software