Which one is Claws on Linux using?
The essence of this discussion is that the problem is only with IMAP access.
... However, the error rate has gradually crept up over time and now seems to be happening approximately 70% of the time I send a message (not on the phone, just when using Thunderbird on the PC).
So, I'm still confused as to what is causing the problem and whether it is due to my home PC set up or something to do with the way the BT broadband and BTYahoo servers are handling things.
The saga continues. An interesting development (but unfortunately not a solution).
The relative who has Sky broadband has recently been moved across from the Sky-Google to the Sky-Yahoo e-mail service. Sky informed all of their customers a few months ago that this would be happening behind the scenes. I assume Yahoo just offer a cheaper service than Google. Anyway, he has told me that he is now getting exactly the same "failed to save copy in sent folder" error every now and then. Like me, he has used IMAP for years but has never had this issue with the Sky e-mail service before. To me this does imply it's some underlying problem with the Yahoo IMAP service. Interestingly, unlike BT, Sky do say they fully support IMAP.
I have had this problem consistently for over a year now and quite frankly I'm tired of it. At least 50% of my attempts to send a message from my PC generate this error. The BTYahoo IMAP service just isn't good enough and BT seem to have absolutely no interest at all in officially supporting IMAP.