I set the "Save copy of outgoing messages to this folder instead of Sent" option on a folder residing on the imap server. When I sent a reply to a mail in that folder the imap server had problems and claws-mail tried to move the mail a few times but then gave up but did not save the email locally. Losing mails is an inconvenience that I'd like to avoid, so please fix this. Thanks in advance.
This is unavoidable if your folders are remote and the server stops working. Currently only one "sent" folder per account is supported, so what you're asking is a new feature currently not available, which could be sumarised as "allow a fallback local sent folder when remote sent folder is not available" or "allow a secondary sent folder per account" (could also be useful for local accounts if partition where primary sent folder lives becomes full). Marking as feature request.
I'd like to see at least a big warning window that the server had problems and emails got lost (the window should contain a list which mails were affected). Call it a "feature request" or "bug report", but I consider this to be critical. Given that the mail was saved locally at first, I think the loss was avoidable by simply keeping the mail were it was (in the queue/draft folder). But this might be discussing semantics. So in any case, please avoid losing mails whenever possible. Even if it is by simply dumping the mail into the log file or something like that.