What are Clusters?
BlueMail organizes similar emails together to remove a lot of clutter from your inbox, without hiding any emails. It categorizes emails from known senders into smart clusters, and organizes your emails into a sub-folder structure automatically, without the hassle to manage it manually.
Any email that comes into a cluster acts as a reminder for the previous emails you might have missed, so as soon as a new email is received from that service or person, the cluster pops up to the top of your inbox.
You can customize your notification settings per cluster by tapping More… | Notifications | Global Notification Mode | People & Specific Services then tap on Specific Services.