An email client is a software that provides a user interface to read emails that it fetches from a mail server. People are quite used to online email clients such as Gmail or Outlook.com but some prefer desktop email clients that are installed on the users and not in a browser.
Let's take a look at the desktop email clients that i've used:
Email clients installed in a GNOME Desktop Environment
It's one of the most popular emails clients across all platforms and a stable version has been available since quite a long time. I think it looks dated and I'm not a big fan of the threaded view. When I get bored of other email clients, I will come back to Thunderbird as the safe, secure and stable option.
$ apt install thunderbird
Geary is a very simple and lightweight native email client. Its made for GNOME and looks beautiful from a distance. I think it lacks customization options. For example, I couldn't change how the font looks for read/unread email to something that would be visually different. I couldn't toggle between dark/light modes and it wouldn't convert links to links when pasted.
$ apt install geary
Previously known as Nylas N1, is based on electron and it's a beautiful email client. Obviously, not very lightweight because well, electron. I found it to be a nice balance between features and good looks. The bad thing about Mailspring is that it requires you to have a mailspring id and not all of it's components appear to be open source.
$ wget https://updates.getmailspring.com/download?platform=linuxDeb