The need to completely remove Exchange can have many reasons, I for one went through this exercise in my lab recently, and while it should be the last hope it is technically possible and the process is relatively easy too.
To completely remove Exchange from your forest open ADSIEdit and connect to the configuration node:
Navigate to the CN=Configuration=, DC=YourDomain, DC=Name and then to Services and delete the Exchange and Autodiscover nodes :
Next connect to the Default Naming Context which will be DC=YourDomain,DC=Name and delete the Microsoft Exchange Security Groups and Microsoft Exchange System Objects containers:
The last step would be removing the various users that are created as part of the Exchange installation, these users will vary from installation to installation but can be summarized as follows:
If you already removed the servers on which Exchange was installed, or where the installation failed, you are good to go if you still need to cleanup data on the Exchange servers there is still some work you need to do which I’ve summarized below.
Exchange server disk
Completely remove the installation directory (by default C:Program FilesMicrosoftExchange Server)
Exchange server registry
Remove the ExchangeServer key which can be found under HKLMSoftwareMicrosoft
Remove-Item -Path HKLM:SoftwareMicrosoftExchangeServer
And the MSExchange key which can be found under HKLMCurrentControlSetServices
Remove-Item -Path HKLM:CurrentControlSetServicesMSExchange
Exchange server IIS
From II Manager you will need to delete the two sites BackEnd and FrontEnd websites .
That’s it once you’ve deleted all the above data all references to the Exchange organization will be removed form your Active Directory deployment and you can proceed with a fresh Exchange installation.