I'm using Outlook Express (6, I think) on Windows XP. Lately, it has been hanging. Sometimes this happens at startup (right after the main window with all the panes loads) and sometimes when selecting many emails (sometimes as low as three emails at once, sometimes at ten, it's not a fixed number). When this happens, msimn.exe starts to use 98-100% CPU and RAM usage shoots up very quickly, reaching hundreds of megabytes in half a minute. The message pane goes gray instead of showing the message contents.
As I said, this sometimes happens right after the main window loads, sometimes when selecting many emails at once. I tried backing up everything, deleting the identities, creating a new one and restoring, but this still happens.