Mailspring vs Polymail

Mailspring is a desktop email client for Linux, macOS, and Windows, with support for multiple accounts in one inbox. It connects to Gmail, iCloud, Office 365, Fastmail, Outlook.com, Yahoo!, and IMAP/SMTP providers, and indexes email for advanced Gmail-style searches across connected accounts. The client works offline and supports scheduled sending. Its free features include undo send, spell and grammar checking, custom themes, dark mode, and localization. Pro adds read receipts, link tracking, send later, follow-up reminders, templates, contact profiles, snoozing, and mailbox insights. Mailspring connects to mail providers through IMAP and to calendars and contacts through CalDAV and CardDAV, though calendar tools are not included in the app. It can translate drafts into Spanish, Russian, simplified Chinese, French, and German, and offer to translate incoming messages. Mailspring says credentials are kept in the system keychain and email is not processed in the cloud. The free plan is available; Pro costs $8 a month, billed monthly.

Polymail has no write-up on Specifiction yet; its figures are above.