Basically the heading says it all! I don't know what I did, but for some reason when I right click on a message, "open in conversation" only comes up in the Inbox folder. It's not there when right clicking on messages in other folders, and is greyed out when I check under "More" under messages options (beside "Delete") when messages are in other folders. The fun thing about this is that I moved the one message I wanted to see in conversation back to the Inbox, and this option reappeared. So it's definitely to do with the folders, not the message itself. But as I have three years of email on this account, moving anything I want to see "in conversation" back to the inbox folder would be very inconvenient and isn't really a solution. Any help is appreciated!