Well, if you prefer a phone with a large screen -- then some areas will be difficult to reach with one hand (top, or bottom, or sides...)
My personal everyday phone is an LG G3 (and before that, other 5.5"+ models), and well, yes, I can't reach all corners of the screen if I'm holding it with one hand... not talking about Aqua, but about all apps.
It's just a given. A compromise, a price you pay for that larger screen. Personally, I'm fine with that, I like having a larger screen for web browsing, and so the compromise, to me, is worth it.
Even with your BlueMail screenshot -- the list of accounts is near the *top*, so it's a stretch (compared to if it was near the *bottom*).