I found that Dovecot's IMAP capabilities are not being reported to clients, because of it being fronted by imapfront.
This can easily be resolved by adding an environment file, I've posted a fix on my contribs site (see sig).
I don't know how many IMAP clients are sensitive to knowing server capabilities, but Mulberry definitely is, e.g. it can use server-side sorting & threading, but only if the capability is reported!
I'd be interested to know if anyone finds this improves behaviour of any other IMAP clients.