A couple of open source clients [1] and [2]- both self-hosted and deployable on AWS or any other cloud service, or any other hosting service with ssh root access:
[1] https://squirrelmail.org/about/ [2] https://www.horde.org/apps/webmail
As an aside, VestaCP [3] - an open source hosting admin panel that you can install within a few minutes via ssh - will install squirrelmail automatically: