Mail Server works with other programs in collaboration  to make up what we know as Messaging System. Messaging system must work without any flaws to keep smooth movement of Inbound and Outbound emails. Mail System uses following protocols to send and receive email messages.

SMTP (Simple Mail Transfer Protocol) – For Sending emails

POP3 (Post Office Protocol 3) or         -  For Receiving emails.

IMAP (Internet Message Access Protocol)

At any point you found out that emails are not reaching their destination, there are high chances of these emails would get stuck in Mail Queue. Only way to release emails from mail queue is to restart mail services. Restarting mail services is done from Mail Server. Make sure before restarting mail service you ask anyone who has good knowledge of doing it.

Windows Server 2003 provides improved alternatives to CDONTS. To make CDONTS
functioning on a Windows Server 2003-based computer, use one of the following

solution:

1. Obtain CDONTS.DLL and put it into %systemroot%\system32 folder
(C:\Windows\system32 by default).

2. Register the CDONTS.DLL component on your server using the following command:

regsvr32 “%systemroot%\system32\cdonts.dll”

For example:

C:\WINDOWS\system32>regsvr32.exe cdonts.dll

3. After you have registered your CDONTS.DLL component you need to check whether
your SMTP service is running. Go to Administrative tools > IIS and expand your local
machine. If SMTP service is listed then it is installed, else  it’s necessary to
install this service. To install SMTP perform the following operation:
Go to Control Panel > Add/Remove Programs > Add/Remove Windows Components >
Application Server > IIS > Mark SMTP service > Click OK.

4. Change the port number for SMTP service. Default port is 25. Use 25 only if you
don’t have another SMTP service running. If another SMTP service is already running
on your server you should switch IIS SMTP port to another, for example to 26. You
can do through IIS management console:
Control Panel > IIS > expand `local computer` > SMTP > Properties > General tab >
click on Advanced button > Edit.

5. Configure SMTP service. The main things are to set a valid full-qualified domain
name for SMTP service:
Control Panel > IIS > expand `local computer` > SMTP > Properties > Delivery tab >
click on Advanced button

6. Configure Security for SMTP service. It’s necessary to grant permissions to
IIS_WPG standard IIS Worker Process Group.

Control Panel > IIS > expand `local computer` > SMTP > Properties > Security tab >
click on Add button > Cick Object types… button > Mark Groups item > click OK >
Add IIS_WPG as object name to add > Click OK.

7. Additional setting:

Choose Authentication and tick Anonymous Access and Integrated Windows
Authentication. Click OK, then click CONNECTION. Configure RELAY settings as you
wish. Click the DELIVERY tab then click ADVANCED.

8. Stop SMTP service of IIS and start it again. Now IIS SMTP service as configured
and ready to work.