While using DarwinMail you will notice that we have a Pinned, Starred & Snoozed labels which can be applied to your emails.
All three of the above are custom DarwinMail labels (which are created when you first log into DarwinMail) that provide the following functionality.
Pinned is for emails you want to keep in your Inbox until you have dealt with them as they are high priority.
They cannot be marked as done or sent to the trash until the Pinned label is removed.
Pinned emails exist in your Inbox and in your Pinned label - which is accessible by clicking the switch at the top of your Inbox next to the search bar.
Starred is accessible via the sidebar and is for emails you don't want to forget about but they are not your priority right now.
Snoozed allows you to hide emails until a later time and date, at which point they will reappear in your Inbox.
PS: While Gmail implements similar functionality, they do not expose those operations via their API and so we have created bespoke solutions in DarwinMail to provide the same functionality and more.