Cannot delete messages from Inbox in KMail

Help
1

I’m new to KMail, and I’m currently experiencing a roadblock with trying to delete messages from the inbox in KMail 6.6.2 (25.12.2) and would like to ask for some help.

The problem is, that Edit ▸ Move to Trash (or Move to Trash from the context menu, or Del ) are disabled for the Inbox folder. Deleting messages is working fine for, e.g., Archive which means, that the trash folder is correctly configured at

Configure ▸ Accounts ▸ Receiving ▸ My Server ▸ Modify ▸ Advanced ▸ Trash folder

as My Server/Inbox/Trash

The folder structure that is displayed in KMail is as

My Server
  Inbox
    Sent
    Trash
    Drafts
    Archive
    Spam

The IMAP server used is dovecot and the namespace is configured as

namespace inbox {
  separator = .
  prefix = INBOX.
  inbox = yes

  mailbox Sent {
    auto = subscribe
    special_use = \Sent
  }
  mailbox Spam {
    auto = create
    special_use = \Junk
  }
  mailbox Drafts {
    auto = create
    special_use = \Drafts
  }
  mailbox Trash {
    auto = create
    special_use = \Trash
  }
}