MailSpring earlier known as Nylas Mail or Nylas N1 is an open-source email client. It saves all the mails locally on computer so that you can access them anytime you need. It features advanced search which uses AND and OR operations so that you can search for mails based on different parameters.
The easiest way to install Mailspring on Ubuntu 18.04 is by using snap. Open up terminal and enter:
$ sudo snap install mailspring