One thing which worked very well for me is focusing only on one thing per email. Startups always have this conflict of trying not to spam their users too much but still making sure they know everything. The outcome is that every time they do send emails they try to include e-v-e-r-y-t-h-i-n-g in it. I struggled with it a lot as well, at one point we decided to dedicate each email to one new small feature, asking for a certain feedback, etc. Even when we released a new version we focused only on one thing. I think users find it much easier to read and digest. It also increases the chances your users will understand what you want them to do :)