I havnt had windows xp crash on my computer ever, I dont know what you are talking about.
Using DOS for emails would be stupid, difficult to do, and a waste of space.
I'm fairly sure you either can't have dos and XP on the same drive, or you can but it's a ******* to do.
You'd need a different email client, dont know what one you'd use.
Dos is by no means "uncrashable", windows 95,98, and ME use a dos base kernel and they crash all the time. If you do manage to use it without crashing its just because you don't have software conflicts since only old software will install, and you dont have anything corrupt.
Trust me XP is a massive improvement on dos.
And I'm pretty sure stuff line the space stations and such used Unix, which linux is based on.
If you want something with a command shell, use some version of linux, red hat's the easiest for beginners.