Another mail server is probably running in your computer. The SMTP port 25 is used to receive email from the internet.

To find out which process is using the port 25, you can do the following:

Windows
open the command prompt and type:

netstat -noa

this will display something like:

Active Connections

  Proto  Local Address          Foreign Address        State           PID
  TCP    0.0.0.0:25             0.0.0.0:0              LISTENING       3888

(and many other lines like this)

3888 is the process ID of the application using port 25

You can find that application by its ID in the Windows Task Manager, which can be displayed by pressing Ctrl+Shift+Esc. To make sure that the PID column is displayed, check View/Select columns from the menu.

Linux

as root, tpe the following command:

lsof -i:25

this will give you the PID of the process using port 25.

You can find more about the process with the command:

ps -ef

DeskNow can be used as a standalone mail server, or as frontent for another existing mail server. If you want DeskNow to receive mail from the internet, it must use port 25. Otherwise you can keep your existing mail server, and use DeskNow as a front-end. This can be be set in the configuration wizard. It is also possible to integrate DeskNow with the most common Unix MTAs, like Postfix, Qmail, SendMail, etc. See this article for an example.