Since you responded elsewhere that your client stores mails in a proprietary format, one very important product goal should be an export mechanism that can export to one, if not more, of the common formats (mbox, maildir, etc.). After all, if you’re confident on your client’s features as the USP to get customers, you should also easily allow those who want to move out to do so. I personally wouldn’t try our client for serious use otherwise.

Of course, I haven't put my focus on this yet, but it is on the todo list.