SquirrelMail

Strengths and Limitations

Squirrelmail is a fully featured email client that will perform most tasks you expect from any email program.
It is very mature software with a familiar interface, and can be expected to work very consistently from one day to the next, across any number of browsers. In situations where you are using an underpowered computer, or extremely slow connection, you will find squirrelmail works better than Round Cube

However, it does not support CalDAV/CardDAV, which means it cannot be configured to sync with those advanced hosting services. It also performs as a web page, meaning you need to click links to make stuff happen