If your ISP blocks port 25 your Exchange server(s) will be unable to send SMTP email. This article explains how to work around this in Exchange 2007. Other versions of Exchange will be similar.
If you have an Edge server, like I do, you cannot make the configuration changes directly on the Edge server. You have to make them on a Hub Transport, CAS or Mailbox server. The changes will then replicate out to the Edge server on the next EdgeSync.
Here's what to do:
- Open EMC and navigate to Hub Transport under Organization Configuration.
- Select the Send Connectors tab and double-click the send connector for outbound email to the Internet.
- Click the Network tab and select "Route mail through the following smart hosts".
- Click Add to add your ISP's outbound SMTP server (i.e, smtp.comcast.com)
- Click the Change button to configure Smart Host Authentication Settings.
- Select Basic Authentication, and enter your ISP account's user name and password.
- Click OK twice to close the dialog windows.