We use Google accounts for our email. But when you are on an airplane, or in some remote location in the bush without internet access, it’s useful to have an offline solution for your email.
I’ve tried out several different alternatives including Apple Mail, Gmail Offline, Outlook, and Thunderbird. Spark is by far the best option out there. Of course it only does email, so if you want your calendar offline as well, Apple Calendar is a decent option. Although these days I just use the Google Calendar app on my iPhone.