I'm building a simple webmail and I would like to make use of the CONDSTORE extension for IMAP: it allows to fetch everything that has changed (messages, flags, …) since a date, which is very practical to synchronize the email client to the IMAP server.
However I have absolutely no idea if this IMAP extension is actually supported by most IMAP servers or not.
So is there any kind of source that could tell me how well this extension is supported? For example, does GMail or Hotmail support it?
(the hidden question behind this is obvious: is CONDSTORE the most appropriate way to synchronize my email client to the IMAP server? I store locally copies of emails, that's why I need synchronization)
No, it is not widely supported. You can treat it as an optimization to use when available, but you cannot depend on it. You also need a way to track removed messages, which is not something condstore can tell you
Gmail, however, just rolled out support for it recently.