Have you tried the Roundcube interface?
It's at:
https://runbox.com/roundcube/
I have grumbled about the Runbox web-mail interface in the past - there's a long thread here:
http://www.emaildiscussions.com/showthread.php?t=67376
But this was started before the current version was installed and before Roundcube was made available. I still like Roundcube but I am now in a position where I can use an IMAP client on my own laptop most of the time, so I don't use it very often. If you haven't tried it, it's worth looking at.
Rgrds F