I've been using GFI Faxmaker since around version 10, but mostly in Exchange environments, and usually trying to get it to work with a variety of analog modems.
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I have a customer that's moving to a hosted Exchange solution, and we've just moved the last of the users mailboxes off-site. Now came the simple task of pointing the Email2FAXGateway settings to send mail to the hosted servers.
We don't use GFI Faxmaker to send, so won't have to resort to any special TextAPI formatting, outlined at GFI's support site.
We initially performed a test, sending inbound fax messages directly to the hosted servers, and it seemed to work well, but a few days later noticed there was a problem. I think what was happening was the hosting mail servers performed an RDNS (reverse DNS) lookup, which fails, since mail sent out of GFI appears to be coming from faxmaker.com.
If I'm not mistaken, you have to have IIS SMTP services installed on your fax server, but if you don't, just use Add/Remove Windows Components to install it.
Once you've got that bit done, you'll need to set up relay access for your local machine. You can find that by locating the Default SMTP Virtual Server, right-clicking, and choosing Properties. Check the Access tab, and click the Relay... button. Add your local hosts IP address, and any others that may need to use this as an SMTP relay.
Then right-click the Domains object beneath the Default SMTP Virtual Server container, and choose New, Domain. Add an alias for the faxmaker.com domain, and another for the smsmaker.com domain.
Lastly, add one for your external domain name, but make this one Remote. In the properties for the domain that you just created, click to select the Allow the Incoming Mail to be Relayed to this Domain check box.
Click the forward all e-mail to smart host option, and then type the FQDN of the server that is responsible for e-mail for that domain in square brackets.
That worked for me!