I’ve been using Runbox for a couple of years and agree the web interface is lacking. There were also multiple significant server outages last year, but it has been much more reliable this year. As long as you don’t use the webmail often, Runbox is a good affordable email provider and is great for families due to the low cost of sub accounts (additional users).
I have been using it for around 10 months now. The first couple months were great, but now it loses connection multiple times per week and that is only resolved after rebooting the modem. There was also a two week period that they were working on the tower and resulted in me having no internet during that time. It’s definitely annoying, but the only other option I have is CenturyLink DSL. If I had fiber or cable internet available, I would use that instead, but the slow DSL speeds make it worth putting up with T-Mobile’s unreliability.