My IMAP folder structure looks like the following:

user@domain.com
├ Inbox
. ├ Sent
. ├ Trash
. ├ Drafts
. ├ Subfolder 1
. ├ Subfolder 2
. ├ Subfolder 3

Messages residing in the Inbox folder cannot be moved to Trash. The "Move to Wastebin" toolbar icon is greyed out.

Messages residing in subfolders such as Subfolder 1 can be moved to Trash. The "Move to Wastebin" toolbar icon is available.