Unfortunately some IMAP clients simply don't get on well with some IMAP servers. That is the one real downside of IMAP, it is a very complex protocol (more complex than it needed to be in my opinion). POP3 on the hand could hardly be less complex.

Copying large amounts of data from one folder to another tends to highlight any such issues that the client/server combination has.

Until Dovecot came along getting a reliable server that worked well with a range of clients just didn't seem possible. Thankfully Dovecot made that a lot better, though still not enough to make any of Microsoft's IMAP clients usable when not run against Exchange's IMAP implementation

I'd be surprised if Dreamhost weren't using Dovecot, I've got no experience of how OSX's Mail.app talks to Dovecot, but I do know at least that iOS's Mail.app works well against it.