Spark is an email app by Readdle for personal and team use. It connects Gmail, iCloud, Outlook, Yahoo, Exchange, and IMAP accounts in one inbox that syncs across devices.

Smart Inbox - automatically sorts personal email, newsletters, and notifications into groups.
Focus tools - Priority and Pin highlight important mail, Group by Sender bundles threads, Gatekeeper screens new senders, and Set Aside, Snooze, Send Later, and Reminders manage follow-up.
AI assistant - drafts and refines emails, translates messages, and summarizes long threads.
Team collaboration - shared drafts, private comments on emails, and delegation with deadlines.
Calendar - built-in calendar with meeting notes alongside mail.

It is used by individuals managing multiple accounts and by teams handling shared inboxes. There is a free download; paid plans add premium features, with details on the pricing page.