For those who are curious about its history, this is where it all began for the visited links to be that color by default.

Date: Mon, 12 Apr 93 02:38:29 -0500

Subject: NCSA Mosaic 0.13 released.

• Changed default anchor representations: blue and single solid underline for unvisited, dark purple and single dashed underline for visited.

Other people already mentioned Roundcube and Snappymail, which are good options already.

There’s also Cypht if you want a different approach. It combines multiple accounts into same interface so you can have a unified inbox.