Error deleting message while moving message to mailbox '(null)'

[#1]

Tried to set up a brand new iMac for a relative (running MacOS 10.11.1 ootb.)
He is with Plusnet too, so I followed the Plusnet help page for connecting Apple's mail.app
I used IMAP.
The messages downloaded fine, but when he came to delete the spam, the mail app popped up
a dialog box stating

Error deleting message while moving message to mailbox '(null)' 

with every attempt to delete a message - even though the messages then disappeared.
No help on Google, so I deleted the account by deleting the folder

~/Library/Mail/V3

and tried POP3 instead.
Fixed.
So what is wrong with the Plusnet IMAP implementation that causes this cryptic error message on a clean system ?
Now, I'm quite happy to believe that the Apple unboxing experience often falls apart like this for even the mildest edge cases, but mail.app has been around for over 15 years - you'd have thought they would have a solid IMAP client by now...
Anyone else seen this on a brand new 2016 10.11.1 El Capitan system ?
I'd like to switch him back to IMAP, but can't until I know why this is b0rked.

[#2]

Nothing is wrong with the Plusnet IMAP implementation, I suspect it's confusion between local folders and server folders on the iMac that's causing the error.
The only Apple device I have is an iPad mini running iOS 9.2.1; the email settings in MacOS 10 might be different. On the iPad to get folder behaviour correct I needed to go into the mail settings and display the Account settings. Scroll to the Advanced option at the bottom of the list and display that. The top section showed Mailbox Behaviours and I set Drafts Mailbox, Sent Mailbox and Deleted Mailbox to be the appropriate folders *on the server*. In the Move Discarded Messages Into section I ticked Deleted Mailbox.
I think I restarted the iPad after making those changes "just in case", but after restart email handling worked as desired.
Hope that gives some clues for what might be needed on MacOS 10.

[#3]

Thanks for the reply,
The relative also has an iPad running iOS 9 which I successfully set up with Plusnet IMAP about 6 months ago - I didn't have to make any changes to advanced settings - it just worked.
I think mail.app must be quite different to iOS mail despite the OSX team trying to mimic some of the appearance of the iPad/iPhone version - I'm not sure where those settings would be.
Some Googlefacts suggest selecting the menu that contains the Trash folder and selecting a sub-option to use it as Trash (?woo-woo alert ?) - but all the main mailboxes are greyed out and they have (read-only) in the window title.
I'll put it down to atrocious Apple software engineering and move on...