I think you need to specify the path to the mailbox in the name, like
"dxbgen/jhon".
You also say the mailbox is "level one", are you sure it is contained
by the account.
Try this:
Applescript:
------------
--this mailbox is at level one
set parent_mailbox to "dxbgen"
set child_mail_box to "jhon"
tell application "Mail"
--trying to create the new mailbox
-- Check to see if this mailbox does not exist.
if not (exists mailbox (parent_mailbox & "/" & child_mail_box)) then
-- If it doesn't exist, make it.
make new mailbox with properties {name:parent_mailbox & "/" &
child_mail_box}
end if
end tell
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