[CentOS] off topic question plss need ur help

Dear All,

I have been using centos 5.1 for quite some time as out DNS and mail server and been workin fine

i use sendmail as my mail server

i did setup a backup mail server as per docs and ran th dns test from http://www.checkdns.net

and it says both the mail servers are fine

but i would like to know the following

how would i really know if my main primary server is down all my mails r goin to my secondary mail server

and when my primary is up are the mails copied back to my primary

i did put down my mail server for abt 3 hrs cause cant keep it down for long but did not really see anything

also are there any websites that can chec both my mail servers correctly

apprecite and thnaks