I'm trying Missive for the first time and it seems to be a great app. Really thinking in change from Polymail.
Two feature that a noticed Missive is missing and I think is very useful is:
1. Inbox category like in gmail web in order to filter email like newsletter and updates from social networks to other inbox and the main email in a focused inbox.
2. Filter in the inbox like outlook or gmail so we can filter by unread messages for example.
Please look at this and keep doing the good job.