This procedure will work for Imail versions 8.2x-10.01.

First, you need to know where the domain folder currently resides. This can be seen by selecting the domain in IMail Administrator and reading the path listed in the Top Directory field. 

Next, shut down all IMail services and move the data to the new location. The path will need to be updated in IMail Administrator and the services restarted.

Locate the folder for your domain in IMail Administrator using the above information.

Shut down any running IMail services. You can leave IMail Administrator open.

Create a copy of the directory from step 1 and place it in the desired location. For example, copy the 'mail_domain_com' folder from C:\IMail to D:\IMail.

In IMail Administrator, enter the path to the new folder for the domain and click Apply. When prompted to go to the file manager and move the directory, click OK.

Restart the previously running IMail services and close IMail Administrator.

NOTE: If running 2006 or newer, you will need to set all the permissions on the folder in the new location so that IIS can access this without error.