EM Client
eM Client is a desktop email client for Windows and macOS. It features sending and receiving email, managing calendars, tasks, contacts, and notes, and XMPP-based communicators. It was originally released in 2007 and still receives regular updates as of August 2023. Versions for Android and iOS were released for beta testing in 2023. Features 
eM Client has a range of features for handling email, including advanced rules management, mass mail, delayed send, or a built-in translator for incoming and outgoing messages. It supports signatures, Quick Text, and tagging and categorization for easy searching. Watch for Replies and Snooze Email functions are available, as well as direct cloud attachments from cloud services like Dropbox, Google Drive, OneDrive, ownCloud or Nextcloud. eM Client provides also a lookup service for GnuPG public keys (eM Keybook) in order to more easily send encrypted communication via email, and generally simplify PGP encryption in email communication. eM Client supports all major email platforms including Exchange, Gmail, Google Workspace, Office365, iCloud and any POP3, SMTP, IMAP or CalDAV server. Automatic setup works for Gmail, Exchange, Office 365, Outlook, iCloud, or other major email services. Recently, auto-import option was added to transfer data from IncrediMail. Since version 8.2, eM Client supports online meetings via Zoom, Microsoft Teams, and Google Meet). eM Client allows extensive appearance customization. References

Email clients
Personal information managers
Calendaring software