Hi Kevin, thanks for your response.
I seem to remember reading somewhere that eM Client can not create certain folders (e.g. Sent) on the Plusnet server because they are regarded as “top level” and therefore protected.
Update: I’ve found this article on one of the Plusnet Community pages:
https://community.plus.net/t5/Email/Settings-for-eM-Client-getting-error-messages/td-p/1526930
I don’t know if that throws any more light onto the subject, or just muddies the waters!
Anyway instead of using IMAP, I finished up using POP3, which seems to work OK for me. I can choose to leave messages on the Plusnet server or not.
Although I’m still not sure why my iPhone manages to write to my Plusnet “Sent” folder (as observed by looking at Plusnet webmail) but eM Client can’t.
Not the end of the world I suppose, but just a little annoying sometimes.
Possibly not the answer you were looking for, but I hope it helps a little.