The hardest bit when building a client is that keywords are per-folder, so unless you store all the email in a single folder server-side, you have to run the ke

The hardest bit when building a client is that keywords are per-folder, so unless you store all the email in a single folder server-side, you have to run the keyword search in every folder, every time, and you can't get updates to multiple folders at once, so you have to re-run that search frequently, which hammers servers pretty hard.