To distribute mailboxes alphabetically, you need to use the If <property> <relation> <value> condition. Select the Last Name property in the first box, matches regexp in the second box, and specify the corresponding regular expression in the third one. For example, to include users with surnames that start from A-F, specify the following: ^[a-f].
Make similar action sets for all mailbox stores specifying the above condition with the corresponding letter sequence for every of them.
For more details, see tutorial Automate Exchange Mailboxes Creation for New Users.