I've installed the tool, but when the main server comes back online the second dhcp server does not shut down.

I'm using Windows 2003 standard R2 SP1, on both servers.

Any idea?

Does it start automatically when the main service goes down?

It might take up to 60 seconds before it shut down the second service when the main comes up.

This could happen because of a bug in version 1.0 when the DHCP-Redundancy service could not connect to the mailserver when it tried to send the alert mail.
Upgrade to version 1.1

It could also be that you have installed it on a localized version of 2003 Server. Version 1.11 supports English and Swedish.

I have 2 Windows 2003 Std Servers. When main server went down, 2nd server kicked in and DHCP service was starting ok.

However, when primary DHCP service working again, It doesn't stop 2nd server DHCP server service...

However, Primary server is also mail server as well.. so I am thinking this could be reason?

Please try the new version 1.22.
What language are there on your servers?
What messages do you see in the Event-log?

I have tried 1.22... However it is doing same. No stopping DHCP service on 2nd server after 1st server recover.

I am using English Windows 2003 Std Server, and I only have this warning log.

Event Log : 105
"The DHCP service has detected that it is running on a DC and has no credentials configured for use with Dynamic DNS registrations initiated by the DHCP service. This is not a recommended security configuration. Credentials for Dynamic DNS registrations may be configured using the command line "netsh dhcp server set dnscredentials" or via the DHCP Administrative tool. "

then,

Event Log :1044
"The DHCP/BINL service on the local machine, belonging to the Windows Administrative domain nsit.local, has determined that it is authorized to start. It is servicing clients now."

then,

"The DHCP Server service entered the running state."

that's it.. it's not sending DHCP server service stop control..

Are both DHCP-services authorized in the Active Directory?