The "four squares surrounded with a dotted line" is the standard Android "select all" icon.
It seemed a bit confusing to me too, at first, and only mention that I took it from Android sources because hopefully it'll ring a bell with at least some users -- whereas I feel that an all-original design would not.
The "mark read" is an open envelope, and "mark unread" is a closed envelope. Done form scratch for my app by someone else, but the idea is taken from the native Android Gmail / Email apps. Again, may not very obvious, but I'm hoping for some recognition from at least those who've used those apps before.
Finally, and most importantly: those icons can be long-pressed to show a floating "tooltip" with their "title".
A standard Android 4.0 feature, which I emulate on Android 2.0, but darn, it took me something like six months from when I got my first 4.0 device to find this out.
( as an aside, I find the discoverability of recent Android UI patterns to be very poor -- interactive UI elements are often made to not stand out from non-interactive ones, and you'd supposed to guess to swipe, or scroll, or pinch, or something, to use them... )
