> Well, could you add instant messaging functionality to IMAP?
IMAP does not reimplement XMPP, which I consider a feature. If you want instant messaging, you should use XMPP, and it's perfectly possible for a client to allow you to use both.
> Can you manage multiple inboxes with one IMAP account?
Yes, this is built in to the standard and every client can do this
> Can you do server side threading with IMAP?
Yes: http://tools.ietf.org/html/rfc5256
> Can you manage attachments and messages separately with IMAP?
Admittedly not, but again do you really want to? There are separate protocols for managing files and we already have dropbox.
> Can you upload attachments NOT using base64 using SMTP/IMAP?
Yes: http://tools.ietf.org/html/rfc3516
> Can you use labels with IMAP?
Google do it this way, but it's not standardised: https://developers.google.com/gmail/imap_extensions#access_t...
There are also user-defined flags
> Can you manage multiple domains under IMAP?
Yes, almost every client supports multiple accounts.
IMAP is a good candidate for the most often (and most badly) reinvented protocol around. There's plenty of reasons to expose parts off the current infrastructure over HTTP (Mandrill is really useful) but I think if your plan is "reinvent ALL the things" you need to be a lot clearer about what exactly is so backwards about on of the most well studied and well engineered pieces of infrastructure around.