Here is my hopeful setup with starter set... Use a motion sensor on mail box to notify me when I get mail....
Whether or not that works will largely depend on how far your mailbox is from your hub, as well as how much the intervening walls, the mailbox enclosure (metal?), etc interfere with signal transmission.
Use another to turn on bathroom lights when someone enters....
Since the SmartThings Starter Kit only includes one motion sensor I presume you have acquired (or are planning on acquiring) a second unit.
Questions:. Can I ask Alexa if my mail arrived?.
Sure. You can ask her anything. The real question is: Will she give a useful response?
The short answer is, "Yes, if you set things up right." The longer answer involves all of the SmartApp installation and configuration (in both the SmartThings and the Echo app) that will be required to accomplish that. It's not that difficult if you're technically inclined and at least somewhat familiar with the two platforms, but it's definitely not plug-and-play. Like I tell everyone, the place to start is here:
https://community.smartthings.com/ Pay particular attention to the thread titled "UPDATE: Recent SmartThings User Experience & Platform Performance" so that you get an understanding of the current shaky state of the ST environment. You're going to need to adopt a bit of a hacker mentality, as quite possibly more patience than you might already have.
Will the lights go out when I exit the bathroom?
Probably not right away. The problem is that most motion sensors (including ST's) do exactly that; they detect motion, not whether you are entering vs exiting the area they cover. So using them to to trigger events (like turning on a light) when you first enter an area is pretty easy. But it's much trickier to trigger an event (like turning the same light off) based on an absence of motion unless you set up some sort of rule (in a SmartApp, in this case) to trigger said event after a specified period of time during which the sensor does not detect motion. And even then you face problems with having the lights turn off while you're in the bathroom and either not engaging in any detectable motion (y'know, like sitting down) or simply out of view of the motion sensor's detection arc for a period of time >= the "lights off after
n minutes/seconds of no motion" threshold you've defined.
There are multiple ways to skin this particular cat, but each has its shortcomings...so you need to do a fair amount of reading up on how others are doing it in order to determine which method will most likely work best for you.