I think Postbox (https://www.postbox-inc.com https://www.postbox-inc.com) has all of the features that you’re looking for; Mailspring (https://getmailspring.com

I think Postbox (https://www.postbox-inc.com https://www.postbox-inc.com) has all of the features that you’re looking for; Mailspring (https://getmailspring.com https://getmailspring.com) has most.