THANK YOU for a most helpful and near instantaneous reply.

My own experience of building an email server is zero - as must already be evident

tho' I'm quite at home on client system software

Your suspicions match my own in terms of probable causes,
but I haven't yet declared "user error" as my conclusion
My post was groping around for logging evidence to support the case.

So far I have only seen good quality screenshots of the issue

Your post clarifies my next steps when I get on the user's RoundCube system

It's unlikely we'll need server access permissions and logging, 

but if we do, I now have a clue how to proceed.

Thank You again for your help

spilly80