IMail - How to enable SMTP AUTH only connections on an alternate port (RFC 2476)
Question/Problem: I want to enable my users to send emails to my SMTP service on an alternate port, but I only want to accept emails from clients that have authenticated on that port.
Answer/Solution: To do this, you will need to make changes to the following values in the registry. All values are located under HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\SMTPD32\Parameters.
To enable SMTP to listen on an alternate port, set the data for EnableExtraPort to 1.
The default alternate port is 587 (as recommended by RFC 2476), but if you wish, you can specify a different port by changing the data for ExtraPort. Be sure to enter the new port number in decimal and not hexadecimal.
Once you have forced authenticated on the alternate port, until the email client successfully authenticates, IMail will only accept the following commands:
If any other command is sent, IMail will respond with "530 user must authenticate on this port".
Note for Outlook and Outlook Express users: If you tell your client to use SSL over SMTP and set the port to anything other than 25 the client will attempt to negotiate the SSL connection before sending the EHLO as if it was communicating over a dedicated SSL port. This will not work on the extra port. Users must issue EHLO and then STARTTLS to use SSL over the alternate SMTP port.
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