Apple

Engineering has provided the following feedback regarding this issue:

It's: make new mailbox at the beginning of mailboxes of mailbox "Bar".  You're missing the "mailboxes of" part.

Please let us know whether the issue is resolved for you by updating your bug report.

I’ve sent them a script that fails to create ANY new mailboxes. If anyone would like to test it and report back, I’d appreciate it.

Regards

Santa

tell application "Mail"

activate

try

make new mailbox with properties "Year 2016"

say "Mail box Year 2016 created."

on error errmsg

say "Mail box 2016 try one failed to be created"

end try

try

make new mailbox at beginning with properties "Year 2016"

say "Mail box Year 2016 created."

on error errmsg

say "Mail box 2016 try two failed to be created"

end try

try

make new mailbox at beginning of mailboxes of mailbox "On My Mac" with properties "Year 2016"

say "Mail box Year 2016 created."

on error errmsg

say "Mail box 2016 try three failed to be created"

end try

try

make new mailbox at beginning of mailboxes of mailbox "On My Mac" with properties "Year 2016/11"

say "Mail box Year 2016 created, sub mailbox 11 created"

on error errmsg

say "Mail box 2016 11 failed to be created"

end try

try

make new mailbox at beginning of mailboxes of mailbox "Year 2016/11" with properties {"2016 11 17"}

on error errmsg

say "Mail box 2016 11 17 failed to be created"

end try

end tell

References:
	>Re: applescripting sub mailboxes in Mail (From: Brian Christmas <email@hidden>)