After a successful purchase you can download the license file. You can also download the license file when you get the order confirmation. The email of the standard version looks like this:
For Mail Archiver Pro every user has to do the registration. For the Pro version you will get an additional email 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 Pro versions the server needs to be registered, too. Mail Archiver does the registration when the server administration is opened. The server license can be installed in the admin area when the server does't run:
Alternatively, you can install the license manually: From the registration email 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.