...than I thought.
OK, after thinking that I was happily using Thunderbird I found a couple of niggly glitches:
1) It has not imported the Sent Mail boxes from my Webmail and Yahoo accounts, only from Gmail;
2) I can't seem to save attachments, even using the "text with attachments" option;
3) That's all so far
So, I wondered whether Outlook does the same job. - And is that now just called "Mail" in Win 10?
- - - Updated - - -
OK, it seems that this happens with all POP accounts - no import of Sent Mail boxes, only Inbox.
(Dunno what happened with Gmail, then???)