yi.org and secondary mx

[Jacob Schmude]

Hi
	In the yi.org account editor, it mensions a secondary mx. What is
this and what does it do? Does it mean secondary mailbox? Can I, for
instance, redirect email to my ISP address when it's unable to reach my
machine directly through yi.org? I'm online as much as possible, but if
I'm not online in linux, and mail comes to me, what happens? I guess if
it's for no longer than 4 hours it's ok, otherwise it bounces? I'm a
newbie to this dns stuff.