Since a vast majority of questions we get are IMAP related, let me just come out and say it. We have an issue with IMAP 🙁
Basically we do not fully support IMAP namespaces. According to Wes Salmon, our awesome SDET in Messaging, some servers use delimiters that we do not recognize, but would if we fully supported namespaces. This causes some issues with not being able to sync some folders (eg. Sent items).
I know this isn't what you wanted to hear, but I guess we messed up!