Telnet disconnects while testing Port 25 SMTP

I have Windows Server 2008 R2 operating system on the server. The server is not running any Active Directory and is not a part of any Domain. The problem which I am facing is, I cannot run any hMailserver program on the computer. The remote mail client does not connect and hence I tried to use Telnet. While using Telnet is shows that the server is connected to port 25 SMTP successfully but when the telnet is used on client machine, it get disconnected immediately. The hMailerserver log shows that the telnet request was received and responded to, but the connection was disconnected. Therefore If anybody knows why Telnet disconnects while testing Port 25 SMTP then please share some information about the topic. For additional information, all the client machines are running on Windows 7 operating system. Thanks in advance.

Are there any firewalls running on the computer? If yes then it could be the reason behind the Telnet session getting disconnected. All the firewalls have a tendency to block all the unknown traffic originating or terminating on the computer. Due to this tendency sometimes the traffic desired by the user is also blocked which gives rise to the problem similar to the one you are facing. Try adding the process of the hMailserver program to the known applications list of the firewall and then see if you are able to connect using Telnet.

I think there is some problem with the network configured due to which you are facing such problems. There is an application known as Wireshark which gives logs of all the events on the network. Install the Wireshark on the server and then see all the errors when the telnet connection is tried to establish. It will give exact errors while the data packets are being transferred between Client and the server machine which will help you in diagnosing the problem with your network.

Thanks for replying to the post really fast. As suggested I tried to move the process of the mail server to the white list of the firewall but still the telnet session disconnected frequently. Also I installed Wireshark on the server ti monitor all the logs of the data packets and found out that while using telnet, the system gives error saying "Telnet session disconnected due to improper header". How to solve this problem? Please help.

The default IP version of Windows 7 is IPv6 while the default IP version of Windows 2008 server is Ipv4. Therefore when the computer running on Windows 7 is connected on the network, it causes protocol mismatch. Hence you will have to disable the IPv6 support of Windows 7 inorder to make the network work properly. To disable the Ipv6 support, go to network connection and then open the TCP/IP properties. Now remove the check mark from the "Enable IPv6 support" and save the settings. Reboot and see if the telnet is successful or not.

You will have to set the priority of the server higher than the mail server to make the telnet connection work. To make the priority of the server higher, Set the priority of the local IP address of the server higher than the hmailservers autoban level. Once done, restart the firewall with Automatic Protection. If the firewall does not restart then reboot the whole system. After rebooting the telnet connection will work.