What is Reddit's opinion of Pantheon Mail?
From 3.5 billion Reddit comments

➔ Pantheon Mail website

By popularity on Reddit, this Service is:

#69 in Email Client apps

1 review of this app found across Reddit:

The Elementary OS fork is called Pantheon Mail (or simply, Mail).

I've been using Elementary for a while now and absolutely love how beautiful and responsive everything is.

It's also worth noting Mail can be installed on other distros, not just Elementary.