1. Is this something you host, or something I host? 2. How are you handling searching existing mail, tech wise? 3. Does this start to slow down after you have 10k emails in it?
If this supports custom domains and is plugged into decent spam filtering, I'd take a closer look at it.
I initially had a prototype, which used Dropbox-as-a-Database. Basically I stored all emails and metadata (labels, etc) in a private dropbox in JSON-format. It actually worked, but the performance was so bad, that I ditched this approach.
2) PostgresSQL comes with a great full-text search. It's not perfect right now, but it does it job.
3) Honestly, no idea :)
Spam filtering is done by MailGun. CustomDomains are a must, of course.