I encountered this while reviewing bug 1211261. Tracing out, the issue is that by skipping the call to MoveNewlyDownloadedMessages at nsParseMailbox.cpp line 2066, m_filterTargetFolders does not get the target folder added, and CallFilterPlugins is never called.
In some cases, it becomes impossible to move a message to a maildir local folder. If a filter contains such a move, the filter fails silently.
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