This problem was reported in the past. But the bug report was closed because of un-reproducability. #4206 "Unreliable mbox export, frequently fails to export messages" https://www.thewildbeast.co.uk/claws-mail/bugzilla/show_bug.cgi?id=4206 The point is that in some cases the exported mbox file is empty (0 Bytes) in a special case. Steps to reproduce: 1. Close claws mail if open. 2. Open your claws mail. 3. Have a folder with minimum(!) 506 mails in it. 4. Export it to an mbox file. -> The Result is file with some KB of size. 5. Repeat step 4. -> The mbox file is empty. This bug is also related to #4307 "MBOX export limited to 505 mails per folder" https://www.thewildbeast.co.uk/claws-mail/bugzilla/show_bug.cgi?id=4307
works for me under linux. Not able to test with windows 10, so leaving open but reverting back to "Claws Mail (Windows)" and "Windows 10" until it can be verified there.