[Users] How to remove e.g. 'sent' folder from Mailbox

[tech-info]

Hello,

i use Claws Mail 3.13.2 (Linux Mint 18) with a couple of
POP-Mailaccounts. Every single one has its own Mailbox (MH).
So i got 15 times the 'trash' , 'sent' , etc. folders.

Since a couple of hours I try to get rid of the unused 'sent' folders.
(I direct the sent mails into the 'inbox' folder.)
And I would like to delete a couple of 'outbox' folders.

But i can not figure out how it works. When i 'right-click' on the
folder the delete option is greyed out.
Even deleting the folder from the Filesystem oder messing with the
folderlist.xml (Is there any manual on this?) didn't help.

I would love to get a tipp how it works or where i could start
exploring the needed Infomation.

Greetings,

Gerd