In the past 3 months I have setup accounts on all three of FastMail, HushMail, and ProtonMail. I was mainly interested in having an account with a custom domain. I used the domains I got through Amazon AWS Route53. ProtonMail was the only one I could not setup custom domain on. It would get stuck in the TXT field verification step. I contacted their support and they were not interested in helping me figure out what's going on. They just told me that was AWS problem and I needed to contact AWS support. I cancelled that account. I only used it for about 3 weeks, and my impression is that it was least polished (web UI and iOS app-wise) of the 3. FastMail was the best.