website and mail managment

Hi,


I have 2 domains and i would like to know a few things...

IF i decide to run all mail trough my mailserver would I
get than into trouble with the hosting trough my ISP??
so in other words if i point the mail to my static IP
adress would this have consequences for my website hosted
by my ISP??

I see I have the possibility to change only mail in my
domain providers, access panel...

what if my mailserver is NOT on (poweroutage, problems
with telephoneline and so on???))???
do I loose my mail than or what happens??
I have now Xdsl and normally I should be on 24/24...

Thanks

Peter