After a successful purchase you can download the license file. You can also download the license file when you get the order confirmation. The mail of the standard version:
For Mail Archiver Pro every user has to do the registration. For the Pro version you will get an additional mail where you can download the license for the main application and the server.
Select "Register" from the application menu. Select the license file to open it.
Alternatively, you can do this from the Registration tabsheet of the Preferences. Drag the registration file to the window or click the Register button and select the file. You will be able to see the information of the registration file in the window.
If the license file is valid, you will see the following dialog.
For both version of Pro you also need to register the server. From the registration mail download the server license. Copy the license into Library/VServer64/Licenses and restart the server. This needs to be done only once on the computer where the server is installed.