(1) If I try to apply message filters to a folder (in maildir format) that is missing some message files, the operation fails without any proper indication of what caused the problem.
(2) When I try to view the filter log, it shows up in a small window with a large font. I cannot read the log properly within Thunderbird, and there’s no indication of where to find the filterlog.html file. I found it by digging around, and it displays much better in a web browser.
(3) The “Match all” or “Match any” conditions don’t seem to be very flexible. For example, I haven’t found any way to specify that a rule should run if “sender is firstname.lastname@example.org AND subject contains any of the following: dog, cat, hamster, goldfish.” I’d like to find a way to include a long list of phrases to match.