I made a few AppleScripts already to get the list of mailboxes out of Outlook. I know that many users have a lot of mailboxes. Like hundreds or mailboxes. Some mailboxes may have a low number of mailboxes and some may be even empty. We are going to modify the AppleScript we had to include only empty mailboxes.

I can count the number of emails in each mailbox by getting "count every message" from the mailbox. And then I only add the mailboxes to the list if NoOfEmails is equal to 0. You can easily modify the AppleScript to include mailboxes with mailboxes when you replace the "= 0" with "> 0".

Here is the relevant section of the AppleScript:
       
set NoOfEmails to count every message of currentFolder
       
if NoOfEmails = 0 then
           
set FolderName to name of currentFolder
               --get full path of mailbox
               
set end of AllFolders to my replace_chars(PathSoFar & ":" & (FolderName), "::", ":")
           
end if
       
end if

The full AppleScript is below:

set AllFolders to {}
tell application id "com.microsoft.outlook"
   
set allMailFolders to get mail folders
   
repeat with currentFolder in allMailFolders
       
set NoOfEmails to count every message of currentFolder
       
if NoOfEmails = 0 then
           
set FolderName to name of currentFolder
           
set PathSoFar to ""
           
if FolderName is not missing value and (container of currentFolder is not missing value or account of currentFolder is not missing valuethen
               
set ContainerName to "x"
               
set theContainer to currentFolder
               
repeat while ContainerName is not ""
                   
set ContainerName to ""
                   
try
                       
set theContainer to container of theContainer
                       
set ContainerName to name of theContainer
                       
if ContainerName is missing value then
                           
set theAccount to account of theContainer
                           
if theAccount is not missing value then
                               
set AccountName to name of theAccount
                               
set PathSoFar to AccountName & ":" & PathSoFar
                           
end if
                       
else
                           
set PathSoFar to ContainerName & ":" & PathSoFar
                       
end if
                   
end try
               
end repeat

               
set end of AllFolders to my replace_chars(PathSoFar & ":" & (FolderName), "::", ":")
           
end if
       
end if
   
end repeat
   
return AllFolders
end tell

on replace_chars(this_textsearch_stringreplacement_string)
   
set AppleScript's text item delimiters to the search_string
   
set the item_list to every text item of this_text
   
set AppleScript's text item delimiters to the replacement_string
   
set this_text to the item_list as string
   
set AppleScript's text item delimiters to ""
   
return this_text
end replace_chars

Below is the result in Script Editor:

Empty mailboxes from Outlook

The first mailbox from the script is "Auf meinem Computer -> Posteingang" which is really empty:

Empty mailbox in Outlook

And yes, verification is part of developing code. Ahem - I had used "> 0" instead of the "= 0" that I wanted. And therefore, my original result was incorrect.