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(FAQ) Users Receive "5.7.1 550 Unable To Relay" Error When Attempting To Send Mail

Suddenly none of our users can send outbound email but nothing was changed in MDaemon. What is causing this?

This can be caused by Microsoft's SMTP service starting up and binding to the mail ports before MDaemon is able to do so. This may occur either after installing IIS and selecting the Simple Mail Transport Protocol service or after a reboot if that service is installed and not disabled. The Simple Mail Transport Service will start and bind to the SMTP port, which will prevent MDaemon from being able to receive SMTP mail. Users may receive an error that the server does not allow relaying.

To determine if the Microsoft SMTP service is started on the MDaemon server, open a command prompt and type:

Telnet 127.0.0.1 25

Press Enter and if the response contains 'ESMTP Microsoft...' the problem is that the Microsoft SMTP service is running.

To disable the service, follow these steps:

  1. Click on Start
  2. Click on Settings
  3. Click on Control Panel
  4. Click on Administrative Tools
  5. Click on Services
  6. Locate Simple Mail Transport Protocol
  7. Right click on the service
  8. Click Properties
  9. Under Startup type select Disabled
  10. Click Apply
  11. Click OK
  12. Stop the service

You may now restart MDaemon so that it can bind to the SMTP ports.